INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM MOLLUSCA

CLASS POLYPLACOPHORA, OR LORICATA

Exclusively marine, slow-moving animals inhabiting mainly the littoral zone, with only a few species descending to greater depths. About a thousand species are known in total, three of which occur in the Black Sea.

Mostly small animals; the smallest barely reach 1 cm, while the largest grow up to 30-40 cm. The dorsal surface is usually colored in ochre-yellow or brown tones, often adorned with spots of all colors of the rainbow.

Anatomy. The body is typically elongated-oval, flattened dorsoventrally, and consists of a weakly defined HEAD, trunk, and FOOT. Most of the dorsal surface is covered by a shell composed of eight articulated plates (Fig. 203).

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Fig. 203. External Anatomy of the chiton Tonicella marmorea (adapted from Ivanov et al.): a - dorsal and ventral views: 1 - first shell plate; 2 - marginal zone of the mantle; 3, 4 - IV and VIII shell plates, respectively; 5 - head; 6 - Mouth; 7 - mantle; 8 - mantle furrow; 9-11 - genital, excretory, and anal openings, respectively; 12 - osphradium; 13 - gills; 14 - foot

The head is located on the anterior ventral part of the body and is separated from the foot by a transverse furrow. The foot occupies most of the ventral surface and features a large, flat sole. The trunk fills the dorsal region of the body; it is covered by a mantle that lines the entire trunk and wraps around the ventral side, forming a circular fold that borders the head and foot. On the dorsal side, the mantle is covered by shell plates, while the peripheral dorsal area left exposed by the shell forms the marginal zone of the mantle, or perinotum.

On the ventral side, between the head, foot, and mantle, lies the mantle furrow, which houses the mantle organ complex: gills (ctenidia), osphradia, the anus, and paired genital and excretory openings. The shell surface is pierced by numerous pores that lead into branching canaliculi penetrating the entire thickness of the shell up to the dorsal epithelium. These canaliculi contain aesthetes - Sensory Organs unique to chitons. In some species, the shell is reduced or internalized, becoming overgrown by the mantle externally, as seen, for example, in cryptozoans (Cryptochiton).

Integument. The Skin of chitons consists of an epithelium covered with a cuticle and a layer of Connective Tissue. On the gill lamellae, the epithelium is partially ciliated. Glandular mucous Cells are interspersed among the epithelial cells and are particularly abundant on the SOLE OF THE foot. The skin of the perinotum (the marginal mantle zone) is covered by a thick cuticle containing numerous calcareous plates, spines, and chitinous needles.

Musculature is best developed in the foot, consisting of longitudinal, transverse, oblique, and dorsoventral Muscle fibers. The chiton's foot serves as an organ of locomotion and suction, allowing the animal to cling to rocks and hard substrates. The edges of the mantle are likewise penetrated by muscle fibers in all directions, ensuring that when the animal adheres to a substrate, it fits tightly against the smallest irregularities. There is also specialized musculature associated with the Oral Cavity, Pharynx, and radula. In addition, well-developed Muscles connect adjacent shell plates, enabling the chiton to curl up when threatened to protect its soft body parts.

Body Cavity. The gonadal coelom is located in the middle of the body, while the pericardial coelom enclosing The Heart lies closer to the posterior end (Fig. 204). The large cavity containing the viscera is a schizocoel. Parenchyma is predominantly developed in the ventral wall of the head, the marginal mantle zone, and especially in the foot.

Fig. 204. Diagram of the coelom and Reproductive System in chitons (adapted from Dogel):

1 - mouth; 2 - gonad; 3 - Kidney; 4 - gonoduct; 5 - genital opening; 6 - internal opening of the kidney into the Pericardium;

7 - excretory opening; 8 - pericardium; 9 - gills

The Digestive System (Fig. 205) begins with the mouth located on the ventral surface of the head. The pharynx possesses well-developed muscles. A longitudinal muscular ridge - the Tongue bearing the radula - projects from the floor of the pharynx into its lumen. The radula continuously wears down at its anterior end as it scrapes Algae from rocks, while new Teeth are added at the posterior end, which is embedded in an elongated radular sheath. The radular teeth are extremely robust, incorporating iron in the form of magnetite. A pair of small Salivary Glands and a pair of large so-called sugar glands open into the pharynx; the secretion of the latter helps convert dietary starch into sugar. The pharynx leads into a narrow Esophagus, which continues into the midgut. The anterior part of the midgut forms a sac-like expansion, The Stomach, into which the ducts of the large bilobed Liver open. The remainder of the midgut (the intestine) is very long, forming several loops before transitioning into the ectodermal hindgut, which lies beneath the pericardium and terminates in the anus at the posterior end of the mantle furrow.

Fig. 205. Structure OF THE chiton digestive system (adapted from Ivanov et al.):

1 - salivary gland; 2 - pharynx; 3 - esophagus; 4 - stomach; 5 - opening of the liver into the stomach (liver not shown);

6 - midgut; 7 - hindgut; 8 — foot muscles; 9 - renal appendages;

10 - visceral artery; 11 - radula; 12 - sugar gland; 13 - pharyngeal muscles

The excretory system is represented by a pair of relatively large Kidneys (coelomducts) located on the sides of the body, which are shaped like U-shaped, highly branched canals (Fig. 204). One end of the kidney opens via a funnel into the pericardium, while the other opens via an excretory pore at the bottom of the mantle groove just anterior to the anus.

The Circulatory system. The heart is located on the DORSAL SIDE OF the body above the gut and consists of a single ventricle and two lateral atria. The atria communicate with the ventricle through valved apertures that prevent the backflow of Blood. The posterior end of the ventricle is blindly closed, whereas the anterior end extends into an aorta that reaches the head, opening widely into the cephalic sinus. The rest of the circulatory system consists of schizocoelous clefts within the parenchyma—lacunae—and large organized cavities—sinuses. Subsequently, venous blood flows to the afferent branchial sinuses and, after oxygenation via the efferent sinuses, enters the atria, which pump it into the ventricle.

The respiratory organs are ctenidia, situated at the bottom of the mantle groove in a single row on each side (Fig. 203, b). Unlike most Mollusks, their number ranges from 4 to 80 pairs, with one pair lying posterior to the excretory organs being larger than the others. Each ctenidium has a bipectinate structure, consisting of a tapered axis with gill lamellae extending from both sides. The surface of the lamellae is ciliated, and the beating of these cilia circulates Water around the gills, facilitating gas exchange.

The Nervous System (Fig. 207) is represented by a nerve ring from which two pairs of longitudinal nerve cords—the pedal and pleurovisceral cords—originate. The supraesophageal part of the nerve ring is called the cerebral arch (Brain) and appears as a thick cord lacking ganglia. The pedal cords are located within the musculature of the foot; the pleurovisceral cords run through the trunk above the mantle groove and fuse at the posterior end of the body above the anus. The pedal cords are interconnected, as well as connected to the pleurovisceral cords, by numerous irregularly arranged transverse commissures. The circumesophageal ring innervates the head, oral cavity, and intestine; the pleurovisceral cords innervate the gills, mantle, trunk musculature, kidneys, and heart; the pedal cords innervate the foot. Ganglia are also absent along the longitudinal cords. There are only small paired buccal and subradular ganglia associated with the subesophageal portion of the nerve ring, which innervate the pharynx and radula.

Fig. 206. Schematic cross-section of a chiton (after Kilias):

1 - shell; 2 - aorta; 3 - longitudinal muscle; 4 - Ovary; 5 - kidney; 6 - aesthetes;

7 - pleurovisceral cord; 8 - afferent gill vessel; 9 - perinotum muscles;

10 - cuticular spines; 11 - gills; 12 - efferent gill vessel;

13 - pedal cord; 14 - foot; 15 - blood lacuna; 16 — intestine

Sense Organs in chitons are poorly developed, which is related to their sedentary lifestyle. Chemoreceptors (olfactory organs) include a ridge of sensory epithelium located at the Base of the gills on the floor of the mantle groove, and small osphradia situated on either side of the anus, which appear as groups of sensory cells. The ORGAN OF TASTE is the so-called subradular organ—an invagination of the oral cavity lined with sensory cells.

Fig. 207. Central Nervous System of a chiton (after Dogel):

1 - cerebral arch; 2 - buccal ganglion; 3 - nerves to the Oral Cavity and pharynx;

4 - subradular ganglion; 5 - pedal cord; 6 - pedal commissures; 7 - pleurovisceral cord;

8 - commissure of the pleurovisceral cords

The tactile function is performed by structures called aesthetes, which perceive water currents and pressure over the body surface. Aesthetes are located within specialized channels on the dorsal side of the shell plates. Above each aesthete, a cuticular cap forms, covering the sensory and glandular cells. The inner ends of the sensory cells continue into nerve fibers that pass through the canals within the shell and connect with the pleurovisceral cords.

The reproductive system. Chitons are dioecious animals without Sexual Dimorphism. The gonad (ovary or Testis) is unpaired and lies medially along the body above the intestine; paired ducts (oviducts or sperm ducts) extend from it, opening via a pair of genital pores at the bottom of the mantle groove (Fig. 204). Copulatory organs are absent.

Reproduction. Chitons release their eggs directly into the water either singly or in gelatinous cords, while some species deposit them into the mantle groove, which Functions as a brood chamber.

Fig. 208. Development of the chiton Ischnochiton magdalensis (after Dogel):

a - trochophore at the onset of metamorphosis; b - later stage:

1 - prototroch; 2 - shell plate rudiments; 3 - foot rudiment; 4 — ocellus

A trochophore larva hatches from the egg, resembling that of Annelids. It features an apical plate with a tuft of cilia and a prototroch (Fig. 208). It subsequently acquires certain characteristics typical of chitons: the primordia of eight shell plates appear on the future dorsal side, with one developing anterior to the prototroch and seven posterior to it. On the ventral side, the rudiment of the foot develops as a ciliated outgrowth, while one or two pairs of ocelli form below the prototroch. The larva swims in the water Column for a short time, grows uniformly, and then settles to the bottom. The larval organs (prototroch, ocelli, etc.) degenerate; a longitudinal mantle furrow forms along the sides of the body, where the gills begin to develop.

The body gradually acquires the shape characteristic of chitons.

Most chitons inhabit shallow waters, with many species populating the intertidal zone. They cling firmly to stones or rocks, using their foot and the underside of the mantle as a powerful suction cup. If a wave does dislodge the mollusk, it rolls into a ball, thereby protecting its soft ventral surface. This behavior also serves as a defense against predators; instances of chitons being found in the stomachs of fish and starfish are extremely rare.

In the northern seas of Eastern Europe, common species include the marbled chiton Tonicella marmorea, the white chiton Trachydermon albus, and Lepidopleurus arcticus. In the Far Eastern seas, the giant Pacific chiton Cryptochiton stelleri can be found, reaching up to 18 cm in length, in which the mantle completely covers the dorsal surface, concealing the shell plates.

Chitons cannot tolerate significant drops in salinity and therefore predominantly inhabit seas with oceanic salinity levels. Only three species are known to inhabit the Black Sea.

Certain chitons are used as food—for instance, Chiton tuberculatus, Cryptochiton stelleri, and others. Their muscular foot and, in some cases, the "roe" of these mollusks are consumed.