ZOOLOGY OF INVERTEBRATES IN THREE BOOKS - BOOK 2 - G.I. Shcherbak - 1996

PHYLUM ANNELIDA

CLASS POLYCHAETA

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This class comprises over 6,000 species of marine (predominantly) and freshwater (less commonly) animals. A single tropical species, Lycastopsis catarractarum from the family Nereidae, is known to inhabit banana and cocoa plantations, feeding on rotting leaves and sweet fruits. About 200 species have been found in the Black and Azov Seas.

The size of polychaetes ranges from a few millimeters to several meters (Eunice gigantea).

The body of polychaetes (Fig. 2) consists of a HEAD lobe (prostomium), a segmented trunk, and an anal lobe (pygidium). Sense Organs are located on the prostomium: a pair of palps, one or several pairs of antennae, eyes, and olfactory pits. In various representatives, certain Organs of the prostomium may be modified or disappear, especially in burrowing and sessile forms. The prostomium, together with the first or two or three preceding segments (peristomium), forms the head region. The fusion of the anterior segments leading to The formation of the head region is called cephalization. The Mouth and cirri, which function as Sensory Organs, are located on the peristomium.

Fig. 2. External anatomy of Nereis pelagica:

a — anterior end with retracted buccal region; b — with everted region; c — posterior end; 1 - antenna; 2 - palp; 3 - peristomial cirri; 4 - prostomium; 5 - olfactory pit; 6 - peristomium; 7 - parapodium; 8 - eyes; 9 - jaw; 10 - buccal region; 11 - peridium; 12 - pygidial cirri

The trunk consists of a variable number of segments (from 5 to 800).

In most polychaetes, each trunk segment, except for the peristomial ones, bears a pair of lateral outgrowths—parapodia.

Fig. 3. Parapodium of Nereis pelagica:

1 - dorsal cirrus; 2 - notopodium; 3 - aciculae (support setae); 4 - neuropodium; 5 - ventral cirrus; 6 - neuropodial setae; 7 - notopodial setae

A parapodium is an unsegmented outgrowth of the body wall, consisting of a basal part and two branches: the dorsal (notopodium) and the ventral (neuropodium). Chemosensory and tactile organs—the dorsal and ventral cirri, respectively—are located on the notopodium and neuropodium. In many polychaetes, the dorsal cirrus is modified into gills. As a rule, a pair of thick supporting setae (aciculae) is present inside the parapodium, while bundles of thin, rather complex setae of varying length and thickness protrude outward. The setae are composed of a Chitin-like substance. Parapodia move from front to back: by gripping the substrate with its setae, the animal moves forward with sinusoidal undulations, while both parapodia of the same segment move simultaneously in opposite directions. Swimming forms possess paddle-like parapodia; in some, the dorsal lobe disappears. The sinusoidal movement is preserved during swimming. In sessile and burrowing forms, the parapodia are partially reduced or completely absent, as in Polygordius.

The anal lobe (pygidium) in many active forms bears a pair of sense organs—anal cirri.

Externally, the body of polychaetes is covered by a thin layer of elastic cuticle containing Collagen fibers; chitin is absent. The cuticle performs a purely protective function. Beneath the cuticle lies a single-layered epithelium (epidermis), which frequently contains glandular Cells (Fig. 4). These are best developed in sessile polychaetes. Their secretion—a mucous substance—can harden to form a protective chitinoid tube around the body, sometimes impregnated with calcium carbonate. In some forms, part of the epithelium is ciliated, with the cilia forming rings and a ventral band (Protodrilus).

The Muscles are smooth and form two layers: an outer circular layer and an inner longitudinal layer; the latter is typically not continuous, but divided into two pairs of bands—dorsal and ventral (Fig. 5). Near the parapodia, the circular layer breaks up into several Muscle bundles that move the parapodia and setae. Each segment contains a pair of diagonal muscles that obliquely cross the body cavity, attaching at one end to the basement membrane of the epidermis on both sides of the ventral nerve cord, and at the other end near the Base of the parapodia. They also participate in the movement of the parapodia. Within the muscle mass, there are cavities containing the supporting setae of the parapodia, which are connected to the body wall by specialized muscles. The peristomium contains specialized muscles that evert the buccal region (see below) and muscles that retract the Pharynx along with the buccal region.

On the inner side, the musculature of the Skin-muscular sac is closely adjoined by a layer of cells—the peritoneal epithelium, or coelothelium—which lines the secondary body cavity, the coelom (see Fig. 1).

Fig. 4. Diagram of the ULTRASTRUCTURE OF THE polychaete cuticle based on Electron Cell/15.html">Microscopy data:

1 - cuticle; 2 - ellipsoidal bodies; 3 - lamellar layer; 4 - processes of epidermal cells; 5 - mucoprotein layer; 6 - outer layer of the cuticle lacking collagen fibers; 7 - collagen fibers; 8 - pigment granules; 9 - basement membrane; 10 - epidermal cell

The coelom, as previously noted, consists of a pair of sacs in each segment. The left and right sacs enclose the gut and, meeting above and below it, form a double-layered longitudinal partition—the mesentery—on which the gut is suspended. The walls of adjacent coelomic sacs form double-layered partitions between segments, known as septa or dissepiments. The septa often contain pores that connect the coeloms of adjacent segments. In the peristomium, septa are generally absent. The coelomic fluid is transparent and contains amoebocytes. The coelomic sacs of each polychaete segment act as a hydrostatic Skeleton against which the muscles exert force during animal locomotion.

Fig. 5. Diagram of a cross-section through a trunk segment of Nereis:

1 - dorsal Blood vessel; 2 - circular musculature; 3 - ciliophagocytic organ; 4 - dorsal cirrus of parapodium; 5 - its nerve; 6 - setae muscles; 7 - notopodium; 8 - supporting bristle (acicula); 9 - neuropodium; 10 - metanephridium; 11 - ventral cirrus; 12 - excretory pore; 13 - parapodial nerve; 14 - diagonal muscle; 15 - ventral blood vessel; 16 - ventral nerve cord; 17 - vessel carrying blood from the ventral vessel to the parapodium; 18 - inner circular vessel; 19 - parapodial capillaries; 20 - perintestinal sinus; 21 - vessel carrying blood from the parapodium to the dorsal vessel; 22 - longitudinal musculature

The Digestive System begins with the mouth opening on the peristomium, passes through the entire body as a straight tube consisting of the buccal region, pharynx, Esophagus, midgut, and hindgut. At the boundary between the buccal region and the pharynx, many polychaete species possess chitinous jaws (see Fig. 2). During feeding, the buccal region can be everted outward, causing the pharynx to move forward so that the jaws are positioned at the anterior end of the buccal region to capture food. The pharynx is muscular with a narrow lumen. A pair of Salivary Glands opens into the anterior part of the esophagus. The long midgut features folded walls, and its epithelium contains numerous glandular cells that secrete digestive juices containing Enzymes; it is here that food is digested and absorbed. The short hindgut terminates in the anus located on the pygidium.

Polychaetes feed in various ways: some are active predators that consume small crustaceans, various Annelids, Mollusks, fish fry, etc.; others are detritivores. Some use their jaws to grasp and hold prey, while others scrape detritus from the substrate. Sedentary polychaetes feed on small organisms and organic particles driven toward the mouth by well-developed head tentacles (palps).

The excretory organs are nephridia (Fig. 6), always arranged metamerically, in pairs within each segment. The inner end of each nephridium is located in the coelomic cavity of the segment, while the nephridial tubule passes through the dissepiment into the next segment, where it opens to the outside on the lateral side of the body (see Fig. 1). Nephridia vary in Structure. Some polychaetes possess protonephridia, which nevertheless differ from those of Flatworms. Instead of a terminal flame cell at the end of the tubules, they possess several club-shaped cells—solenocytes—each of which projects with its blind end into the coelom, while opening into the nephridial lumen via a channel containing a flagellum. The cluster of solenocyte flagella performs the function of a flickering flame (flame bulb). In many polychaetes, the solenocytes are replaced by a ciliated funnel that opens into the coelom. Excretory organs of this type are called metanephridia. The nephridial tubule is very long, convoluted, entwined with blood capillaries, and surrounded by a compact glandular mass composed of coelomic epithelium.

Fig. 6. Excretory organs of polychaetes:

a — terminal region of the Phyllodoce nephridium with solenocytes; b — metanephridium of Nereis; 1 - solenocytes; 2 - flickering flame;

3 - tubule; 4 - glandular mass; 5 - funnel; 6 - nephridial canal; 7 - excretory pore

Nephridia perform the Functions of osmoregulation and The excretion of dissolved Metabolic waste products.

Studies on metanephridial function have shown that coelomic fluid continuously enters through the funnel. As this fluid moves along the tubule, it undergoes modifications. Certain ions and nutrients are reabsorbed from it into the blood, while catabolic waste products pass from the blood into the fluid and are expelled outward as an aqueous solution.

Often, so-called celomoducts, which convey Germ Cells from the coelom to the exterior, are associated with the terminal part of the nephridium. Unlike nephridia—which are ectodermal derivatives—celomoducts are organs of mesodermal origin. They are found in segments in pairs. Each possesses a wide ciliated funnel, a short duct, and an opening.

Very rarely do polychaetes possess both celomoducts and nephridia, as is the case, for example, in the family Capitellidae (Fig. 7). In most polychaetes, nephridia and celomoducts unite into a single organ that simultaneously performs the functions of eliminating excess Water, dissolved metabolic products, and germ cells from the Organism. Such organs are called nephrimixia and exhibit a diverse structure. In some cases, for instance in *Irmalatifrons*, the celomoduct funnel fuses with the nephridial canal, and both germ cells and excretory products are eliminated through it. In others, the celomoduct funnel and the terminal part of the protonephridium (solenocytes) are situated on the same nephridial canal, meaning they share a common excretory duct (*Alciopacantrainii*). Frequently, the number of excretory organs is reduced (oligomerization), especially in sedentary polychaetes.

Fig. 7. Diagram of the relationships between nephridia and celomoducts in polychaetes:

a — independent celomoduct and metanephridium; b, c — nephrimixia (nephridial walls are shown in black, celomoduct walls are hatched); 1 - germ cells; 2 - metanephridium; 3 - protonephridium

In addition to nephridia, an excretory function is performed by specialized cells located on the walls of Blood Vessels, which are capable of accumulating nitrogenous metabolic waste products (guanine, uric acid salts, etc.). These cells eventually degenerate, their contents enter the coelom, and from there they pass outward via the nephridia. Such cells are called chloragogen cells.

The Circulatory system (see Fig. 5) consists of dorsal and ventral blood vessels lying within the dorsal and ventral mesenteries, respectively, above and below the intestine. In addition to these vessels, a narrow slit-like space—the perintestinal sinus—runs around the intestine between its outer wall and the coelomic epithelium. In large polychaetes, this is replaced by a perintestinal vascular plexus. Another longitudinal vessel extends along the ventral nerve cord (the subneural vessel). Blood flows from posterior to anterior through the perintestinal sinus and dorsal vessel, and from anterior to posterior through the ventral and subneural vessels. The longitudinal vessels are interconnected by metameric circular vessels. Each segment has two vessels: one carries blood from the dorsal vessel to the perintestinal sinus, and from there nutrient-rich blood to the ventral vessel; the other carries blood from the ventral and subneural vessels to the body wall, parapodia, and gills (if present), where the vessels break down into capillaries. In the skin and gills, the blood is oxygenated and then returns to the dorsal vessel and perintestinal sinus.

The walls of the blood vessels are formed by a basement membrane—a dense substance with scattered embedded cells. Major vessels (dorsal, some circular) possess musculature that enables pulsation. Polychaete blood distributes nutrients from the gut to body organs and also serves a respiratory function. In many species, it contains dissolved respiratory pigments—Hemoglobin or chlorocruorin.

In some species, specialized respiratory organs are absent, and gas exchange occurs through the body integument, particularly in the parapodia. Many polychaete species possess gills of various structures: in some sedentary polychaetes, the palps or peristomial cirri are transformed into gills; in most, it is the dorsal cirrus of the parapodia. Gills may be leaf-shaped, pinnate, or bush-like. Blood capillaries run inside the gills, and gas exchange takes place across their surface. As a rule, gills are not located on all segments, but only on specific Regions of the body.

The Nervous System (Fig. 8) consists of the Brain—a paired supraesophageal ganglion—from which circumesophageal connectives extend, encircling the pharynx and connecting above it to a pair of ventral cords running along the entire body. In some polychaetes (*Polygordius*, *Protodrilus*), the ventral cords lack ganglia, with Nerve Cells distributed along the cords; in others (*Aelosoma*), discrete ganglia are barely noticeable; in most polychaetes, each body segment contains a pair of well-developed ganglia. In primitive forms (*Aelosoma*), the ventral cords are widely spaced, the ganglia are connected by transverse commissures, forming a "ladder-type nervous system".

Fig. 8. Nervous System of the anterior body region of Nereis:

1 - peristomial cirri; 2 - olfactory organs; 3 - circumesophageal connectives; 4 - subesophageal ganglion; 5 - ventral nerve cord; 6 - peristomial nerves; 7 - supraesophageal ganglion; 8 - eyes; 9 - palpal nerve; 10 - antenna

In most species, the cords are brought closer together, the commissures are shortened, and the paired ventral ganglia are fused, resulting in the formation of a nerve cord (chain).

Nerves branching from each ganglion of the ventral nerve cord innervate the musculature of the skin-muscle sac and the parapodia of the respective segment. Each ganglion receives processes from sensory Neurons that are part of various receptors (such as Touch receptors). The ganglia of the ventral nerve cord ensure sensory perception and motor responses within their respective segments.

The supra-oesophageal ganglion of polychaetes is a rather complex structure (Fig. 9, a). It consists of three divisions: the Forebrain, Midbrain, and Hindbrain. The forebrain innervates the palps, the midbrain the eyes and antennae, and the hindbrain the nuchal organs. In errant polychaetes, the midbrain contains so-called stalked or mushroom bodies—a pair of compact, mushroom-shaped clusters of nerve cells. These stalked bodies serve as the higher associative center of the brain. In sedentary polychaetes, which lack active foraging behavior, these structures are absent, and the overall brain structure is significantly simplified.

Fig. 9. Polychaete sense organs:

a — sensory endings on the prostomium of Nereis; b — statocyst of Arenicola marina; c, d — cross-sections through the eye

of Nereis and Alciopa cantramii, respectively; 1 - palps with their sensory cells; 2 - antennae; 3 - nerve endings on muscles; 4 - stalked bodies; 5 - forebrain; 6 - midbrain; 7 - hindbrain; 8 - tactile cells of nuchal organs; 9 - nuchal organs; 10 - eyes; 11 - epidermis; 12 - duct; 13 - its vesicle; 14 - statoliths; 15 - cuticle; 16 - cornea; 17 - lens; 18 - sensory rods; 19 - pigment cells; 20 - retinal visual cells; 21 - nerve; 22 - crystalline lens

The supra-oesophageal ganglion acts as the center that receives and processes information from the sense organs, regulates The activity of the ventral nerve cords, and coordinates and integrates the work of the segmental ganglia.

Sense organs are represented by numerous Skin Receptors as well as specialized organs. Organs of touch and chemical sense include palps, antennae, and peristomial, parapodial, and pygidial cirri. They are densely covered with sensory cells whose processes form nerves connected to the ganglia: the palp and antenna nerves connect to the supra-oesophageal ganglion, the peristomial nerve to the sub-oesophageal ganglion (the first ganglion of the ventral nerve cord), and the parapodial and anal nerves connect to their respective segmental ganglia (Fig. 9, a). Additional organs of chemical sense are the nuchal organs—pits on the prostomium surrounded by a glandular epidermal fold; at their base lie ciliated sensory cells whose processes connect to the posterior part of the supra-oesophageal ganglion.

Many species of the subclass Sedentaria also possess paired equilibrium organs, or statocysts (Fig. 9, b). In the lugworm (Arenicola marina), these are located on the sides of the first segment; in sabellids (family Sabellidae), In the second segment; while in some species, statocysts are metameric and occur in several anterior segments, such as in Scoloplos, where they are found in segments IV–XIII. Each statocyst appears as a vesicle lined with sensory cells and connected to the external environment by a narrow canal. Inside the vesicle are concretions known as "statoliths" (or auditory stones). Statocysts perceive water or substrate vibrations, prompting the animal to retreat into its tube. Errant polychaetes lack statocysts.

Almost all polychaetes have eyes of varying structure (Fig. 9, c, d). The simplest eyes are found in errant polychaetes as a cup-shaped ectodermal depression. The lining epithelium acts as a retina and consists of two cell types: photoreceptor cells and pigment cells. The photoreceptor cells continue into nerve fibers that form the Optic nerve, which connects to the supra-oesophageal ganglion. The interior of the optic cup is filled with a transparent gelatinous mass—the vitreous body. In some predatory polychaetes (such as the planktonic Alciopa), the eye is even more complex: it separates from the epidermis to form a closed vesicle containing a biconvex lens and a vitreous body, while the epithelium above the lens becomes transparent to form a cornea. Such an eye is even capable of accommodation; through the action of contractile microfilaments, the lens can move closer to or further from the retina, allowing the animal to focus on objects at various distances (Fig. 9, d). These eyes belong to the direct (non-inverted) type. In errant polychaetes, two or four such eyes are located on the DORSAL SIDE OF the prostomium.

In tube-dwelling sedentary polychaetes, numerous eyes develop on the tentacles, palps, parapodia, and even the pygidium. These eyes only detect changes in light intensity, triggering a withdrawal reflex into the tube. Their structure is similar to that of flatworm eyes and belongs to the inverted type.

Polychaetes are generally dioecious animals, showing no Sexual Dimorphism. Gonads form metamerically in most or several segments along the coelomic walls. Gametes are released into the coelom through ruptures of the coelomic epithelium, where they mature while floating in the coelomic fluid. They exit the body through various means: via body wall ruptures (in Nereis, Eunice, etc.) or sometimes through specialized genital ducts, or paired coelomoducts. In most polychaetes, the funnels of the coelomoducts merge with nephridia to form various types of nephromixia (see Fig. 7). Fertilization is external.

In many species, gonads develop only in a specific region of the body, usually the posterior, known as the epitokous region. It differs sharply from the anterior atokous region: wide swimming lamellae and longer setae appear on its parapodia, its coloration changes, and the gut becomes reduced.

Polychaetes also reproduce asexually. Sometimes Selection/8.html">Asexual and sexual reproduction alternate regularly, a phenomenon known as metagenesis.

Asexual reproduction most commonly occurs via transverse fission (architomy). The separated fragments regenerate a head or a tail region. In species of the genus Ctenodrilus, the body may break apart into several fragments, each comprising one to six segments, with every fragment developing into a new worm. This type of reproduction is unrelated to sexual maturity; upon reaching a certain size, the worm automatically fragments into multiple pieces. In some species of this genus, sexual reproduction is entirely unknown. Budding occurs much less frequently (e.g., in Syllis ramosa). Numerous lateral buds form on the body, from which only the posterior part of the worm develops, while the head forms only after the bud detaches (Fig. 10, a). All of this demonstrates the high regenerative capacity of polychaetes.

Fig. 10. Polychaete reproduction:

A - lateral budding in Syllis ramosa; b - incomplete epitoky in the Pacific palolo worm Eunice viridis; c - asexual reproduction in Autolytus; 1 - atokous and 2 - epitokous individuals; 3 - atokous body region; 4 - epitokous body region

Asexual reproduction in polychaetes is often linked to sexual reproduction characterized by epitoky. Epitoky refers to a drastic change in the external Morphology and internal anatomy of a polychaete or part of it during gamete maturation. There are two types: complete epitoky, where the entire animal undergoes transformation, and incomplete epitoky, where only a body region—usually the posterior—becomes epitokous while the anterior remains atokous. An example of complete epitoky is the body remodeling in Nereis virens, accompanied by an increase in body size and altered parapodial structure. The polychaete ascends from the bottom into the water Column, releases its gametes through body wall ruptures, and subsequently dies.

In incomplete epitoky (Fig. 10, b), there are two distinct patterns. In one case, such as in the Pacific palolo worm (Eunice viridis), the posterior epitokous body region detaches from the anterior part, ascends into the water column to spawn, and then dies. The atokous portion remains on the bottom, regenerates its posterior end, and prepares for the next reproductive cycle. In the other case, entire animals comprising both atokous and epitokous regions leave their benthic habitats and swim near the water surface using powerful paddle-like parapodia, eventually releasing massive amounts of gametes. This phenomenon is typical of the family Nereidae, particularly Nereis pelagica.

The reproductive strategy of species of the genus Autolytus (Fig. 10, c) is particularly noteworthy. Before the epitokous region detaches, a head with appendages forms upon it, and a fully developed, sexually mature individual (male or female)—adapted for pelagic life and complex mating behavior—breaks free. Often, even before the first sexual individual separates, a second, third, and up to 30 sexual individuals form ahead of it, creating a long chain. Eventually, this chain breaks apart, the sexual individuals swim away, and the asexual (atokous) maternal worm remains on the bottom. After releasing gametes near the female, the male dies. The female deposits her eggs into a brood pouch located on the ventral side of her body. She protects her offspring with her own body while they develop inside the pouch. All larval stages take place here, and only fully formed juveniles leave the pouch. During their pelagic existence, females do not feed and die after the young depart.

Cleavage in polychaete eggs is total (holoblastic), most frequently unequal, spiral, and determinate, meaning the developmental fate of each blastomere is precisely fixed from early stages. Cleavage results in a spherical blastula. Gastrulation occurs via invagination or epiboly. The gastrula features a blastopore at the vegetal pole. The blastopore then elongates along one side of the gastrula (the future ventral side) toward its equator, takes on a slit-like shape, and closes completely from back to front. The anterior remnant of the blastopore becomes the mouth, while the anus breaks through at the site of its posterior edge, thus forming a complete through-gut. A similar developmental pattern is found in nematodes (see Vol. 1, p. 294).

Direct development is extremely rare in polychaetes; as a rule, the egg hatches into a trochophore larva (Fig. 11, a). This is a typical planktonic larva that swims using cilia. Its body is more or less spherical or slightly elongated, with an apical tuft of cilia seated on a group of ectodermal cells at the upper pole. This serves as a sensory organ, often accompanied by other sensory structures such as a pair of eye spots, short tentacles, and statocysts. A ring of cilia—the prototroch, or pre-oral ciliated band—runs approximately around the equator of the trochophore, dividing the larval body into an upper hemisphere (the episphere) and a lower hemisphere (the hyposphere). The mouth is located directly beneath the prototroch, and the anus sits at the lower (vegetal) pole. In addition to the prototroch, many trochophores possess auxiliary ciliary bands: the metatroch (or post-oral band) located behind the mouth, and the telotroch situated just in front of the anus.

In the episphere, beneath the apical plate, lies a neural ganglion that innervates the larval sense organs. Meridional and circumferential nerve cords connect to it. The trochophore gut appears as a curved tube consisting of an ectodermal esophagus, an entodermal midgut, and an ectodermal hindgut.

Between the body wall of the trochophore and the gut lies the primary body cavity (blastocoel), containing scattered mesodermal cells (mesenchyme) and muscles. A pair of protonephridia is present in the lower hemisphere of the larva.

Near the lower pole of the trochophore, on both sides of the intestine, lie two large cells—the mesodermal teloblasts; sometimes they begin to divide, forming two mesodermal bands.

After a period of planktonic life, the metamorphosis of the trochophore begins. The posterior half of the larval body elongates significantly and divides simultaneously into several (most commonly 3, 7, or 13) segments, upon which the rudiments of parapodia, setae, or rings of cilia develop. At this time, the teloblasts begin to divide, forming two cords—mesodermal bands lying along the sides of the intestine. If these bands were already present in the trochophore, their cells now undergo intensive division. Subsequently, within each of the mesodermal bands, the cells move apart, and a cavity—the coelom rudiment—is formed.

Fig. 11. Metamorphosis of Polygordius:

a — trochophore; b — its further development; c — adult individual; 1 — nerve trunks; 2 — apical tuft; 3 — apical plate; 4 — prototroch; 5 — metatroch; 6 — muscles; 7 — midgut; 8 — hindgut; 9 — anus; 10 — mesodermal band; 11 — protonephridium; 12 — hyposphere; 13 — foregut; 14 — mouth; 15 — episphere; 16 — coelomic sacs

Most often, the coelomic sacs remain solid in the larva, but in some species, they divide, forming a pair of coeloms in each segment. A larva possessing several segments is called a metatrochophore. Its body consists of a head lobe (prostomium), which constitutes an almost unchanged upper hemisphere of the trochophore, several segments, and a small anal lobe (pygidium)—the region of the lower hemisphere of the trochophore surrounding the anus (Fig. 12). The segments formed in the metatrochophore are called larval segments.

Fig. 12. Larval stages of Nereis:

a — trochophore; b — metatrochophore; c — nectochaeta; d — young benthic form; 1 — episphere; 2 — prototroch; 3 — hyposphere; 4 — mouth; 5 — rudiments of parapodia; 6 — prostomium; 7 — parapodia; 8 — pygidium; 9 — growth zone

In many polychaete species, the metatrochophore transforms into the next larval stage—the nectochaeta, which already exhibits head appendages—antennae, palps, and parapodial lobes with setae; that is, it resembles an adult more closely, yet retains larval segments and leads a planktonic lifestyle (Fig. 12, c). Eyes, antennae, and olfactory organs develop on its episphere. The nerve cells associated with them unite to form the brain (supra-oesophageal ganglion). In the ectoderm of the ventral body side, the rudiments of the ganglia of the ventral nerve cord appear. Subsequently, at the posterior end of the larva, anterior to the anus, a growth zone is formed, anterior to which segments known as postlarval segments are formed one after another (Fig. 12, d).

Fig. 13. Development of the coelom in annelids:

a — frontal section through the growth zone; b — d — transverse sections through the worm at varying distances from the growth zone; 1 — coelom; 2 — mesoderm; 3 — ectoderm; 4 — teloblast; 5 — gut; 6 — primary body cavity; 7, 8 — outer and inner layers of the mesoderm; 9, 12 — dorsal and ventral blood vessels; 10 — mesentery; 11 — ventral nerve cord

In the growth zone, the Cells of the mesodermal bands divide intensively, and paired rudiments of the coelomic sacs separate from them. Each newly formed segment possesses a pair of such rudiments, which grow and form coelomic sacs (Fig. 13). With its outer side, each sac lines the skin-muscular sac of the corresponding segment, while its inner side surrounds the intestine. The walls of the coelomic sacs adjoining each other above and below the intestine form the dorsal and ventral mesenteries, and at the point of contact between sequentially arranged sacs, transverse partitions between segments—septa, or dissepiments—are formed. Blood vessels arise between

two layers of the coelomic epithelium in the dorsal and ventral mesenteries. The musculature of the skin-muscular sac and intestine, coelomic fluid cells, Blood Cells, and chloragogen cells are also formed by the cells of the mesodermal bands. Segment formation in the growth zone continues for a very long time, sometimes until the end of the worm's life, leading to a significant increase in their number.

Thus, polychaetes possess two groups of segments that differ in their mode of formation. Larval segments arise first, all simultaneously, by the segmentation of the lower hemisphere of the trochophore. Postlarval segments are formed successively, one after another, in the growth zone.

In some polychaetes (subclass Myzostomida), the body consists solely of larval segments throughout life. Such animals are called oligamerous. In most, however, postlarval segments are also formed. These are polymerous annelids. In polymerous annelids, gonads, coelomoducts, and nephridia develop only in the postlarval segments.

The Class Polychaeta includes three subclasses: Errantia, Sedentaria, and Myzostomida.