Kinga Molnár1, György Kriska2,3, and Péter Lőw1

1 Department of Anatomy, Cell and Developmental Biology, Eötvös Loránd University, Budapest, Hungary

2 Institute of Biology, Eötvös Loránd University, Budapest, Hungary

3 MTA Centre for Ecological Research, Danube Research Institute, Budapest, Hungary

7.1 Introduction

7.1.1 Taxonomy

An annelid, phylum name Annelida, also called segmented worm, is any member of a phylum of invertebrate animals characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. They are found worldwide from the deepest marine sediments to the soils in our city parks and yards. As a major invertebrate phylum of the animal kingdom, the annelids number more than 9000 species distributed among three classes: the marine worms (Polychaeta), which are divided into free‐moving and sedentary, or tube‐dwelling, forms; the earthworms (Oligochaeta); and the leeches (Hirudinea). Earthworms and leeches are the familiar annelids for most people, but polychaetes make up the bulk of the diversity of Annelida of which most people are completely unaware. They are found in nearly every marine habitat, from intertidal algal mats downwards. There are even pelagic polychaetes that swim or drift, preying on other plankton, and a few groups occurring in fresh water and moist terrestrial surroundings (Rouse 2002).

A recent cladistic analysis of Annelida and other groups has resulted in a new classification of polychaetes (Rouse and Fauchald 1997). The Annelida is found to be monophyletic, though weakly supported, and comprises the Clitellata and Polychaeta. The group Polychaeta is split into two main clades: Scolecida and Palpata. Scolecida is a small group of less than 1000 named species, and these worms are all burrowers of one form or another, with bodies reminiscent of earthworms. Palpata makes up the vast majority of polychaetes and is divided into Aciculata and Canalipalpata. Aciculata contains about half of the polychaete species and largely encompasses the old taxonomic group Errantia. Representatives of this lineage are characterized by having internal supporting chaetae, or aciculae, in the parapodia. It includes major groups such as Phyllodocida and Eunicida, which tend to be mobile forms with well‐developed eyes and parapodia for rapid locomotion. Canalipalpata, a group with more than 5000 named species, is distinguished by having long grooved palp structures that are used for feeding. Canalipalpata is divided into Sabellida, Spionida, and Terebellida. Most of these groups’ members live in tubes and use their palps to feed in various ways.

7.1.2 Life History Life Expectancy

The life span differs greatly among annelids. An earthworm can live for 4–8 years. Living a prolonged life span can be challenging because they are food sources for birds, toads, and rodents, and often are used for home composting, as well as commercial and recreational fishing.

Since both the polychaetes and oligochaetes are able to regrow lost parts (i.e., regenerate), it may appear that they are essentially ageless. Few longevity studies have been carried out with polychaetes, however. Most of the adults of species studied have a characteristic number of segments, which form rapidly during early life and prior to the appearance of gametes. Many polychaetes, especially among the nereids, reproduce only once and then die. In nature, these worms, usually quite sluggish after spawning, are eaten by fish and other animals. Species of polychaetes are known to live from one month (Dinophilus) to three years (Perinereis). Species that form stolons (stems), such as the syllids, or whose posterior end breaks off, such as the palolo, are capable of repeating the process but the number of times and the length of time for which they are able to do so have not been established. Most sedentary polychaetes survive following spawning, but, again, it is not yet known how often this process can be repeated.

The life span of oligochaetes is better established because they are frequently used in laboratory experiments. Some earthworms are believed to live as long as 10 years. Senescence, or aging, is known to occur in oligochaetes; Eisenia, for example, lives beyond a reproductive period with a progressive loss of weight. Aging oligochaetes darken in color, largely because of an increase in pigment deposition. In addition, the metabolic rates decrease and their physiologic processes slow.

Little is known about the life span of leeches. One species of Erpobdella requires a year to reach sexual maturity, after which it lays cocoons once and dies. Another species breeds once a year for two years and dies during the third. Reproduction

Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual, while the parent remains a complete organism. The lifecycles of most living polychaetes, which are almost all marine animals, are unknown. Only a quarter of the studied species have two separate sexes, which release ova and sperm into the water via their nephridia. The fertilized eggs develop into trochophore larvae, which live as plankton. Later they sink to the sea floor and metamorphose into miniature adults. While some polychaetes remain of one sex all their lives, a significant percentage of species are full hermaphrodites or change sex during their lives (i.e., sequential hermaphrodites). Most polychaetes whose reproduction has been studied lack permanent gonads, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn (Rouse 1998).

Reproduction in oligochaetes is primarily hermaphroditic; the number, arrangement, and location of the male and female gonads and their pores vary considerably among the various species. Lower oligochaetes (Microdrili) have one pair of testes and one pair of ovaries in successive segments. Higher oligochaetes (Megadrili) retain the two pairs of testes in segments 10 and 11 and one pair of ovaries in segment 13. Developing sperm are frequently stored in seminal vesicles before transfer to female seminal receptacles (spermatheca). Sperm ducts lead from the seminal vesicles to male pores located one or more segments behind the testes. The ovaries are simple outpouchings (ovisacs), with oviducts leading to female pores in the next posterior segment.

Though these crawlers have male and female reproductive organs, they require mating partners. Copulation in oligochaetes is reciprocal – that is, only sperm is exchanged, but mutually – and takes place in a head to tail position, with the two ventral surfaces in contact. In lower oligochaetes, the male pores of one worm and the female pores of another (openings of seminal receptacles) are opposite each other, and sperm pass directly from the male pores into the seminal receptacle of the partner. In higher oligochaetes genital pores do not lie opposite each other. In this case the seminal fluid is conducted from the male genital pores to the seminal receptacles via paired seminal grooves formed by attaching body surfaces of mating animals. Cells associated with the brain secrete a hormone that stimulates gamete development. The worms separate after the mutual sperm exchange.

The clitellum of the earthworm secretes a case, or cocoon, into which is secreted a material that serves as nourishment for the young and a mucous substance that aids in copulation. The cocoon slips forward and receives eggs as it passes the female pores and sperm as it passes the pores of spermathecae. Fertilization, therefore, takes place within the cocoon. The cocoon slides over the peristome, becoming completely sealed as it does so, and then it is buried. It takes between two and four weeks for baby earthworms to emerge from their cocoon. Diet

The feeding methods of polychaetes are closely correlated with the varous modes of existence the members of the class display. Raptorial feeders include members of many families of surface‐dwelling species, many pelagic groups, tubuculous eunicids, and other gallery dwellers. The prey consists of various small invertebrates, including other polychaetes, which are usually captured by means of an eversible pharynx (proboscis). Not all errant polychaetes that possess jaws are necessarily carnivores. A scavenger or omnivorous habit has evolved in many poychaetes. The jaws may be used, for example, to tear off pieces of algae. Some deposit‐feeding polychaetes consume sand or mud directly when the mouth is applied against the substratum and are generally not selective. Selective deposit feeders lack a proboscis. Special head structures extend out over or into the substratum. Deposit material adheres to mucous secretions on the surface of these feeding structures and is then conveyed to the mouth along ciliated tracts or grooves. These polychaetes thus select organic deposit material from between sand particles. Many of the sedentary burrowers and tubiculous polychaetes are filter feeders. The head is usually equipped with special feeding processes, used to collect detritus and plankton from the surrounding water.

The majority of oligochaete species, both aquatic and terrestrial, are scavengers and feed on dead organic matter, particularly vegetation. The earthworm is an example of a foraging herbivorous annelid, obtaining food by eating its way through the soil and extracting nutrients from the soil as it passes through the digestive tract. Leeches are primarily bloodsuckers. The medicinal leech Hirudo feeds principally on mammalian blood, but it also sucks blood from snakes, tortoises, frogs, and fish; when young, it may eat oligochaetes. The secretion of hirudin facilitates feeding. The leech detaches after becoming engorged with blood and it may not attempt to feed again for up to 18 months. Marine leeches attach to, and feed directly from, the gills of fish. Other leeches are carnivorous and feed on oligochaetes and snails.

7.1.3 Relevance

Marine annelids may account for over one‐third of bottom‐dwelling animal species around coral reefs and in tidal zones. Burrowing species increase the penetration of water and oxygen into the sea‐floor sediment, which encourages the growth of populations of aerobic bacteria and small animals alongside their burrows. The terrestrial burrowers loosen the soil so that oxygen and water can penetrate it. Both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating the decomposition of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily. Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers. Large earthworms (Lumbricus terrestris) are cultivated and sold as bait for freshwater fishes and as humus builders in gardens. The sludge worm Tubifex, abundant near sewer outlets and thus an indicator of water pollution, is collected and sold as food for tropical fish. Polychaetes play an important role in turning over sediment on the ocean bottom. The medicinal use of leeches, which dates from antiquity, reached its peak in the first half of the nineteenth century. The European species Hirudo medicinalis formerly was exported throughout the world, and native species were used. Hirudin, an extract from leeches, is used as a blood anticoagulant. The estuarine flats of Maine and Nova Scotia are the principal sources of the bloodworm (Glycera dibranchiata), which is used as bait for saltwater fishes. Reproductive parts of the palolo (Palola siciliensis), which break off and are found in great numbers during the reproductive period, are used as food in Samoa in the south Pacific.

7.2 Gross Anatomy

Place the earthworm in the dissecting tray after rinsing off the mucus (Figure 7.1). Identify the dorsal side, which is the worm’s rounded top, and the ventral side, which is its flattened bottom. Locate the conspicuous clitellum, a saddle‐like swelling on the dorsal surface (Figure 7.1). The clitellum produces a mucus sheath used to surround the worms during mating and is responsible for making the cocoon within which fertilized eggs are deposited. The anterior of the animal is more cylindrical than the flattened posterior and is the closest to the clitellum. The ventral surface of the earthworm is usually a lighter color than the dorsal surface. The mouth is located on the ventral surface of the first segment while the anus is found at the end of the last segment. Find the anterior end by locating the prostomium (lip), which is a fleshy lobe that extends over the mouth (Figure 7.1). The other end of the worm’s body is the posterior end, where the anus is located. Look for the worm’s setae, which are the minute bristle‐like spines located on every segment except the first and last. Bristle‐like setae are used for locomotion. Find the pair of sperm grooves that extend from the clitellum to about segment 15, where one pair of male genital pores is located. Look also for one pair of female genital pores on segment 14. Try to find the two pairs of openings of the seminal receptacles on segment 10.

Using a pair of scissors, make a shallow incision in the dorsal side of the clitellum at segment 33 (Figure 7.2a). Using a pair of forceps, spread the incision open, little by little. Separate each septum from the central tube using a dissecting needle, and pin down each loosened bit of skin (Figure 7.2b). Continue the incision forward to segment 1. Starting at the anterior end, locate the muscular pharynx (food ingestion) (Figure 7.3). This is followed by a tube‐like esophagus, which terminates in a crop, which serves as a storage organ. Posterior to the crop, the gizzard can be found. While the crop is soft and thin, the gizzard is muscular; soil is ground up and churned within the gizzard. The gizzard is followed by a long intestine in which both digestion and absorption occur. Undigested material is voided through the anus. Identify the five pairs of pseudohearts and find the dorsal blood vessel (Figure 7.3). Look for smaller blood vessels that branch from the dorsal blood vessel. To find organs of the nervous system, push aside the digestive and circulatory system organs and locate the ventral nerve cord. Trace the nerve cord forward to the circumpharyngeal nerve ring, which circles the pharynx. Find one pair of ganglia under the pharynx and another pair of ganglia above the pharynx. The ganglia above the pharynx serve as the brain of the earthworm. The worm’s excretory organs are tiny nephridia. There are two in every segment. The earthworm is hermaphrodite; there are testes and ovaries in every individual. Most parts of the reproductive system are too tiny to locate even with a loupe. The two pairs of spermathecae are observable as small white spheres and the yellowish large lobes of seminal vesicle in the anterior part of the body (Figure 7.3).

Photo depicts lumbricus terrestris in anesthetized state.

Figure 7.1 Lumbricus terrestris in anesthetized state. A, anus; CL, clitellum; CS, contracted segments; DBV, dorsal blood vessel; ES, elongated segments; Per, peristomium; Pro, prostomium.

Photos depict (a) one dorsal cut is enough to reveal the internal organs of the earthworm. (b) Pins in pairs are used to keep the body wall open.

Figure 7.2 (a) One dorsal cut is enough to reveal the internal organs of the earthworm. (b) Pins in pairs are used to keep the body wall open. The intersegmental septa have to be torn to free the internal organs.

Photo depicts dissected Lumbricus in water cover displaying the internal organs.

Figure 7.3 Dissected Lumbricus in water cover displaying the internal organs. CG, cerebral ganglion; ChC, chloragogen cells; CPhC, circumpharyngeal connective; Cr, crop; DBV, dorsal blood vessel; E, esophagus; G, gizzard; I, intestine; IS, intersegmental septa; M, metanephridia; Ph, pharynx; PrN, prostomial nerves; PsH, pseudohearts; OC, oral cavity; Sth, spermathecae; SV, lobes of seminal vesicle; VNC, ventral nerve cord.

7.3 Histology

7.3.1 Body Wall Integument and Muscle Layers

A list of organs for histologic examination is provided in Table 7.1. The body wall consists of the epidermis, connective tissue that underlies it, and muscle layers (Figure 7.4a,b). The epidermis is the thinnest layer within the body wall, except for the clitellar region, where it is thick (Figure 7.4d). Large numbers of unmyelinated axons richly innervate the epidermis. Capillaries enmesh even the epidermis (Figure 7.4d). A fibrous layer beneath the basal lamina separates the epidermal epithelium from the body wall musculature. The connective tissue of the earthworm body wall is largely acellular, but at extremely rare intervals, cells are found embedded among the collagen fibers. Some of these scattered cells are filled with large numbers of phagocytosed bacteria; the rest are characterized by large vacuoles, which contain material that is interpreted as partially digested foreign matter. No cell which appears morphologically akin to the vertebrate fibroblast can be found. The same absence of connective tissue cells has been noted in the connective tissue of the gut and nervous system. Fibroblasts are either absent or extremely sparse in Lumbricus (Coggeshall 1966).

Under the connective tissue, there is a layer of circular muscles and below this is a much thicker layer of longitudinal muscles. The circular muscles contract to make the worm longer and thinner, the longitudinal to become short and stout (Tzetlin and Filippova 2005). The segment walls subdivide the circular muscle layer, while the longitudinal muscle layer is continuous throughout the segments of the animal (Figure 7.4a,b).

The intersegmental septa or dissepiments consist of the extracellular matrix (ECM) situated between the adjoining coelomic epithelia or muscle cells. In addition, blood vessels are formed by gaps within this ECM. The orientation of the muscles can be dorsoventral, oblique, or transverse, or they radiate from the intestine to the body wall (Tzetlin and Filippova 2005). There are radial muscle bundles on the anterior surface of dissepiments and circular ones on the posterior (Figure 7.4a upper right and Figure 7.4b upper and lower right panels) (Pilato 1981). Cuticle and Epidermis

The cuticle of the earthworm consists of collagenous fibers embedded in an amorphous matrix (Figure 7.5b–d). As previous investigators have noted, the fibers are arranged in layers, each layer consisting of a single row of parallel fibers, and the main fiber axis of each layer making an angle of approximately 90° with respect to the fiber axis in the layers above and below (Coggeshall 1966). The outer surface of the cuticle, called the epicuticle, consists of a layer of homogenous, electronopaque material in which are embedded myriad small ellipsoidal bodies. The cuticle of Annelida consists of proteins, which are reinforced by a special type of collagen fiber; in contrast to the cuticle of arthropods, it does not contain chitin. Chitin is present only in the chaetae, which are bristles on the body wall (Peters and Walldorf 1986).

The epidermis of L. terrestris consists of a simple epithelium in the cephalic and terminal regions, and a pseudostratified epithelium in the clitellar region (Figure 7.5a,d). In all epidermal regions are present supporting, basal, sensory, and mucous gland cells. Supporting cells are columnar epithelial cells and they are not specialized as secretory or sensory cells (Figure 7.5a,b). They may contain pigment granules; the apical surface possesses microvilli, which penetrate the cuticle. The nuclei are prominent. Many dark, membrane‐bounded granules, 100–200 nm in diameter, are found throughout the cytoplasm of the supporting cells, but they are most abundant in the perinuclear region. It is generally assumed that the cuticle is produced by these cells. Basal cells featured in the pseudostratified epithelium take part in regeneration (Figure 7.5d). The mucous gland cells (orthochromatic and metachromatic) are mainly present in the clitellar region, while neuroendocrine‐like cells are more numerous in the cephalic and terminal regions (Licata et al. 2002).

Table 7.1 Organs for histologic evaluation in Annelida.

Organ system Organs
Body wall/musculoskeletal Epidermis, muscle layers, setae, clitellum
Digestive Alimentary canal Oral cavity, pharynx, esophagus and calciferous (Morren’s) glands, crop, gizzard, midgut and typhlosole, hindgut
Main blood vessels, pseudohearts, hemocytes
Ganglia, giant nerves
Testis, seminal vesicles, vas deferens, seminal receptacles
Special senses/organs
Photos depict the structure of body wall layers in different regions of Lumbricus. (a) Layers of the body wall and dissepiments at low magnification. (b) layers of the body wall behind the clitellum. (c) Layers of the body wall at the setae. (d) Layers of the body wall at the clitellum.

Figure 7.4 Structure of body wall layers in different regions of Lumbricus. (a) Layers of the body wall and dissepiments at low magnification (median sagittal longitudinal sections, Bouin’s fixation), left panel: 15×; upper right panel: 100×, lower right panel: 150×; (b) layers of the body wall behind the clitellum (horizontal section plane, longitudinal section, Bouin’s fixation), left panel: 15×, upper right panel: 150×, lower right panel: 200×; (c) Layers of the body wall at the setae (anterior end, Bouin fixation), 50× and 300× (inset); (d) Layers of the body wall at the clitellum, (formalin fixation), 100× and 300× (inset). A, anterior; BV, blood vessel; BW, body wall; C, coelom (body cavity); Cap, capillary; CC, coelomocyte; ChC, chloragogen cell; Cut, cuticle; E, epidermis; ECM, connective tissue layer of dissepiment; IS, intersegmental septum (dissepiment); LS, lateral seta; MG, midgut; MLa, muscle layer of intersegmental septum, anterior; MLc, muscle layer of body wall, circular; MLl, muscle layer of body wall, longitudinal; MLp, muscle layer of intersegmental septum, posterior; Mn, metanephridium; MSS, muscles of setal sac; N, nerve; P, peritoneum, Ph, pharynx; PsH, pseudoheart; S, segment; SS, setal sac; T, typhlosole; VNC, ventral nerve cord; arrows in panel (a), suspensory membranes.

In the epidermis of the oligochaete lumbricids, mucous and protein gland cells are commonly found. However, the characterization of the mucous secretion in the two (orthochromatic and metachromatic) cell types has been investigated only with conventional histochemical methods. The orthochromatic (or large granule) cells secrete a complex of neutral mucopolysaccharides, proteins, and lipids (sulfomucins and glycogen are absent) (Figure 7.5a–d). With the light microscope, they appear as a globular cavity filled with PAS‐positive material. Moreover, the dense orthochromatic granules, to which has been attributed a lubricating action during locomotion, may also contain a pheromone important in mating. In contrast, the metachromatic (or reticular gland) cells produce a carboxylated slightly sulfated mucus, probably of low viscosity, which provides a respiratory film held to the surface of the cuticle by epicuticular projections (Figure 7.5a–d). The third type of gland cells, called “small granular cells,” are rich in proteins and are scattered throughout the epidermis (Figure 7.5a,b,d). The secretion of these cells could aid water retention in the acidic mucous film or modify the viscosity of the lubricating mucus. A receptor‐secretory function with paracrine action has also been attributed considering these cells as neuroendocrine‐like, capable of producing bioactive substances (Licata et al. 2002).

Photos depict the morphology of epidermal cell types of Lumbricus after formalin and Bouin’s fixation. (a) Epidermis and subepidermal layers, (b) Epidermal cell types in high magnification. (c) Epidermis and subepidermal layers. (d) Epidermal cell types in high magnification.

Figure 7.5 Morphology of epidermal cell types of Lumbricus after formalin and Bouin’s fixation. (a) Epidermis and subepidermal layers (formalin fixation), 400×. (b) Epidermal cell types in high magnification (formalin fixation), 900×. (c) Epidermis and subepidermal layers (Bouin’s fixation), 400×. (d) Epidermal cell types in high magnification (Bouin’s fixation), 900×. BC, basal cell; BL, basal lamina; Cap, capillary; Cut, cuticle; CF, collagen fibers; CTF, collagen tissue fibers; E, epidermis; LGC, large granule cell; MC, muscle cell; MLc, muscle layer of body wall, circular; MLl, muscle layer of body wall, longitudinal; MV, microvilli; RGC, reticular gland cell; S, connective tissue septa; SC, supporting cell; SGC, small granule cell; SGC1, SGC2, small granule cell, type 1 or 2; black arrow, nucleus of supporting cell; white arrow, nucleus of gland cell; white arrowhead, nucleolus of supporting cell. Setae

Annelids possess chitinous setae (chaetae) that arise from a setal sac (chaetal sac) (Figure 7.6a,b). The setal sac is an ectodermal invagination that generally contains several setal follicles and is surrounded by the subepidermal ECM. Setae are closely paired (Figure 7.6a). Annelids possess segmental pairs of dorsal (notopodial) and ventral (neuropodial) setal sacs, each giving rise to a single row of setae (Tilic et al. 2015). Each seta is formed within one setal follicle, which consists of a basally located chaetoblast and several follicle cells (Figure 7.6b,d). All cells are epithelial, are interconnected by apical adherent junctions (“belt desmosomes”), face the seta and rest on the subepidermal ECM.

Photos depict setae and setal sac of Lumbricus. (a) Setal arrangement: closely paired (Bouin fixation), 20×. (b) Setal sac with two follicles (left side) and distal part of follicle (formalin [left] and Bouin [right] fixation), 120× (left) and 250× (right). (c) Chaetoblast, the deepest follicular cell and the ECM (formalin fixation), 530× (both panels), left: tangential plane, right: longitudinal plane. (d) Setal follicles and their muscles.

Figure 7.6 Setae and setal sac of Lumbricus. (a) Setal arrangement: closely paired (Bouin fixation), 20×. (b) Setal sac with two follicles (left side) and distal part of follicle (formalin [left] and Bouin [right] fixation), 120× (left) and 250× (right). (c) Chaetoblast, the deepest follicular cell and the ECM (formalin fixation), 530× (both panels), left: tangential plane, right: longitudinal plane. (d) Setal follicles and their muscles (formalin fixation), 100×, inset: 40×; horizontal plane, inset: cross‐sectional plane. BV, blood vessel; BW, body wall; C, coelom (body cavity); CB, chaetoblast; ChPZ, chitin polymerizing zone; CG, cerebral ganglion; Cut, cuticle; E, epidermis; ECM, extracellular matrix; Fc, fibrocyte; FC, follicle cell; ICM, interconnecting or retractor muscle of setae; LS, lateral seta; MC, muscle cell; MLc, muscle layer of body wall, circular; MLl, muscle layer of body wall, longitudinal; P, peritoneum; Ph, pharynx; RM, radial or protractor muscle of setal sac; S, seta; (S), fallen out seta; Sec, seta, ectal part; Sen, seta, ental part; SF, setal follicle; VS, ventral seta; arrowhead, brush border; asterisk, radial or protractor muscle of setal sac; black arrow, nucleolus of chaetoblast; white arrow, nucleolus of follicle cell.

There are two types of setae. The general setae are sigmoid in shape with a sharply pointed tip. The other type of setae is associated with genital intumescences. They are called genital setae (or copulatory setae) because they take part in the process of copulation. A genital seta is straight with a grooved ectal part. Depending on the number of furrows, the ectal part of a genital seta could be trihedral or tetrahedral (Csuzdi and Zicsi 2003). Clitellum

The clitellum is a saddle‐like region of modified skin, prominently developed in sexually mature members of the Clitellata (Figure 7.7). The clitellar epithelium contains, in addition to nonspecialized supporting cells, a variety of secretory cells; the latter are responsible for a number of secretions associated with the reproductive processes of the Clitellata. These include the production of the egg capsule (cocoon), nutritive fluid, and the secretion of mucus and substances that aid in adhesion during copulation (Morris 1983).

Photos depict organization of clitellar epidermis of Lumbricus. (a) Morphologically distinct zones of clitellum. (b) Boundary between zone I and zone II. (c) Organization of clitellar glands and gland cells in lateral part of zone II (Bouin fixation, Azan), 330× and 700×. (d) Glands in zone II (Bouin fixation, Azan), 150× and 550×. (e) Medial part of zone II (formalin fixation, HE), 150× and 700×. (f) Boundary between lateral part of zone II and zone III.

Figure 7.7 Organization of clitellar epidermis of Lumbricus. (a) Morphologically distinct zones of clitellum (Bouin fixation, Azan), 30×. (b) Boundary between zone I and zone II (Bouin fixation, Azan), 150×. (c) Organization of clitellar glands and gland cells in lateral part of zone II (Bouin fixation, Azan), 330× and 700×. (d) Glands in zone II (Bouin fixation, Azan), 150× and 550×. (e) Medial part of zone II (formalin fixation, HE), 150× and 700×. (f) Boundary between lateral part of zone II and zone III (Bouin fixation, Azan), 250× and 700×. BL, basal lamina; C, coelom (body cavity); CC, coelomocyte; Cut, cuticle; D, duct of gland cell; E, epidermis; GC, gland cell; LGC, large granule cell; LGCc, large granule cell, clitellar type; MC, muscle cell; MG, midgut; MLc, muscle layer of body wall, circular; MLl, muscle layer of body wall, longitudinal; MSS, middle setal sac diverticula; N, nerve; RGC, reticular gland cell; SC, supporting cell; SGC, small granule cell; SGCc1, small granule cell, clitellar type 1; SGCc2, small granule cell, clitellar type 2; VNC, ventral nerve cord; Zone I, II, III, morphologically distinct zones of epidermis; asterisk, sinus; black arrowhead in panel e, duct of SGCc2; black arrowhead in panel b, c and f, nucleus of supporting cells; dashed line in panel d, boundary between SGCc1 and SGCc2 glands; dashed line in panel b and e, contour of gland cells; white arrowhead, nucleus of gland cells.

The simple columnar epidermis of L. terrestris consists of supporting cells, large granular cells (LGC), and reticular cells, together with the small granular cells (SGC) typical of normal epidermis in the anterior and posterior transition zones. The proteinaceous nature of the granular clitellar cells, evidenced by their histochemistry and the pronase digestion of the granules in ultrasections, indicates that their secretion forms the cocoon wall. In the cocoon wall, parallel‐oriented units are present similar in size to those present in the granules (Hess and Vena 1974). Since the mature granules show that the original secreted subunit (14 nm wide) is altered during maturation and storage, it is presumed that the stored form changes back into structural material at the time of secretion of the cocoon wall. The secretion of the metachromatic reticular cells has a finely fibrous nature and is reminiscent of that of the acid mucopolysaccharide secretion of normal lumbricid skin. The results suggest that the reticular cells secrete the cocoon wall and the globular cells the material that surrounds the developing embryos, the need for a nutritive‐rich secretion in microdriles being obviated by the quantity of yolk present in their eggs (Richards 1977).

Three distinct regions (zones I–III) are distinguishable in the clitellum of the earthworm. This distinction is based largely upon a difference in the glandular elements found in them (Figure 7.7a).

  • Zone I: on the dorsal and lateral surfaces with three kinds of glands. Reticular gland cells (RGC) are typical goblet cells with cytoplasmic mucus. LGC are long, slender, often twisted or convoluted cells extending much deeper into the thickness of the clitellum than the goblet cells and whose contents consist of large granules. SGC are deeper lying groups of gland cells enclosed within envelopes of connective tissue arranged in columns with finely granular contents. Each group is made up of a number of cells, each of which consists of an expanded portion which contains huge amounts of secretory granules and other cell organelles, and a slender elongated portion, which functions as a duct and extends upwards through the thickness of the clitellum until it opens on the surface. The cells are arranged around a central axis, the expanded portions lying in series one above another, and the ducts occupying the center of the group (Figure 7.7b,c).
  • Zone II: the swellings along the sides of the clitellum (tubercula pubertatis). From the region of the lateral setae, the deeper‐lying glands become gradually replaced by relatively larger cells more irregular in shape and less regularly arranged, until by the time the region of the ventral setal pores is reached, the former have become completely replaced by the latter. The contents of these cells are characteristic. The nearer the ventral setae are approached, however, the large‐granule‐containing glands (LGC) diminish in number, and this is often accompanied by a relative increase in the number of mucin‐secreting glands. In the lateral lane of zone II, new types of mucus‐secreting gland cells (stained by aniline blue) appear: the clitellar type of LGCc and SGCc. Two types of the latter can be distinguished on the basis of their Azan staining intensity. SGCc1 (type I) cells are dark blue, and open on the body surface. These cells are part of the lateral side of zone II (Figure 7.7a,b). SGCc1 gland cells are replaced by pale blue SGCc2 (type II) gland cells in the medial part of zone II (Figure 7.7d), which are in connection with the setal pore diverticula. The LGC cells have practically disappeared, and the superficial gland cells – chiefly mucus‐secreting RGC and SGCc1 – are confined to a relatively thin layer simulating more nearly the ordinary epidermis, until it merges into the columnar epithelium of the setal pores (Figure 7.7e).
  • Zone III: the ventral surface of the clitellum. The epidermis itself is less modified and consists of slender columnar supporting cells of the typical epidermis. The subepidermal layer is composed of thin, tall, aniline blue‐positive cells with diffusely granular content. Despite their lighter blue staining, this morphology strongly resembles SGCc1 gland cells. The LGCs and RGC are completely missing from this region.

Between the groups of cells, large intercellular spaces or lacunae are present (Figure 7.7f, asterisk). This condition attains its maximum development in the median line, but at the outer margins approaching the region of the setal pores, the subepidermal tissue, with its lacunae, becomes increasingly replaced by the large cells with lamelliform contents associated with the diverticula of the ventral setal pores. The blood vessels are numerous and occupy the lacunae between the groups of cells (Figure 7.7c). The examination of sections of worms in both these stages shows that the papilla‐like elevations are due to the development of the masses of glandular cells found in relation with diverticula of the ventral setal pores, and that these diverticula are already well established (Morris 1983).

7.3.2 Alimentary Canal

The alimentary canal of Lumbricus is straight and relatively simple. The annelid intestinal wall is composed of an inner layer of mucous membrane (tunica mucosa) consisting of a single layer of epithelium (lamina epithelialis) resting on a delicate loose connective tissue layer (lamina propria) containing blood sinuses or capillaries. The lamina epithelialis always contains mucous gland cells and is typically ciliated. The tunica mucosa is followed by layers of inner circular and outer longitudinal muscle. The intestinal wall is covered by a layer of visceral peritoneum on the outer surface. The relatively extensive foregut, consisting of oral cavity, pharynx, and esophagus (Figure 7.8), is derived from the stomodeum and lined with epidermis, some of which secretes cuticle.

Photo depicts foregut with chromophil cells in midsagittal section (Bouin fixation), 15× and 200× (inset).

Figure 7.8 Foregut with chromophil cells in parasagittal section (Bouin fixation), 15× and 200× (inset). ChrC, chromophil cells; CG, cerebral ganglion; EO, esophagus; Eeo, epithelium of esophagus; F, fold of pharyngeal wall; IS, intersegmental septum; MC, muscle cell; MPh, muscles of pharynx; MPhr, radial muscles of pharynx; NPr, prostomial nerves; OC, oral cavity; Ph, pharynx; SEG, subesophageal ganglion; S1, first segment; S2, second segment (prostomium); VNC, ventral nerve cord. Oral Cavity

The mouth, located beneath the prostomium (see Figure 7.1), opens into a small oral cavity, which in turn opens into a more spacious pharynx. The oral cavity has a simple columnar epithelial lining in Lumbricus. A cuticle covers the epithelium (Figure 7.9). There is loose connective tissue with muscle cells and nerves under the epithelium. The circular and longitudinal muscle layers are very thin and discontinuous; they run immediately under the epidermis (Figure 7.9). Pharynx

The dorsal wall of the pharynx is both very muscular and glandular and forms a pad, which is the principal ingestive organ. In earthworms the pharynx acts as a pump. This action of the pharynx is augmented by the action of strands of muscle fibers extending from the pharynx to the body wall (Figure 7.8). The pharyngeal lining is a pseudostratified columnar epithelium on the ventral side and a ciliated pseudostratified columnar epithelium on the dorsal side (Figure 7.10a,b). Under the thick layer of basement membrane there are chromophil cells, muscle cells, and nerves embedded in connective tissue (Figure 7.10a,b). The pharyngeal “gland cells” of earthworms are not gland cells in the usual sense, and do not communicate with the pharynx. The term chromophil cells is used for them because of their intense coloration by hematoxylin and similar stains. Chromophil cells are situated dorsally on the pharynx, and extend backwards as far as septum 5/6. The so‐called septal glands of earthworms are aggregations of similar cells at a more posterior level (Keilin 1920) (Figure 7.8).

Photo depicts the histology of oral cavity. Oral cavity and its wall (Bouin fixation); 20× and 150× (inset).

Figure 7.9 Histology of oral cavity. Oral cavity and its wall (Bouin fixation); 20× and 150× (inset). BL, basal lamina; BW, body wall; CG, cerebral ganglion; CPhC, circumpharyngeal connective; Cut, cuticle; E, epithelium of oral cavity; MC, muscle cell; N, nerve; OC, oral cavity; SS, setal sac; dashed line, run of basal lamina of the epithelium.

Photos depict the histology of pharynx. (a) Pharyngeal wall and chromophil cells, 20× and 500× (insets). (b) Epithelium and subepithelial layer of pharynx (left panel: ventral side, right panel: dorsal side).

Figure 7.10 Histology of pharynx. (a) Pharyngeal wall and chromophil cells (Bouin fixation), 20× and 500× (insets). (b) Epithelium and subepithelial layer of pharynx (left panel: ventral side, right panel: dorsal side), 600×. BL, basal lamina; BV, blood vessel; BW, body wall; C, cilia; ChrC, chromophil cells; CTF, connective tissue fibers; EPh, epithelium of pharynx; F, fold of pharyngeal wall; LP, lamina propria; MC, muscle cell; MLc, muscle layer, circular; MLl, muscle layer, longitudinal; MPh, muscles of pharynx; MPhr, radial muscles of pharynx; N, nerve; OC, oral cavity; P, peritoneal sheet (capsule); S, sinus; SEG, subesophageal ganglion; SS, setal sac; black arrowhead, nucleus of chromophil cell; white arrowhead, nucleus of epithelial cell.

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Nov 28, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on Annelida
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