INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA (SPIRALIA)

PHYLUM MOLLUSCA (MOLLUSCA)

CLASS APLACOPHORA

This Class comprises exclusively marine benthic animals distributed in all the world's oceans from the Arctic to Antarctica across various depths—ranging from the littoral zone to the greatest depths of the World Ocean (about 9,000 m). They inhabit hydroid and coral polyps colonies, and some burrow into silt. A total of about 150 species are known, with seven found in the Black Sea.

Aplacophorans lack a shell. In appearance, they differ significantly from other Molluscs, which is why these animals were long classified either as worms or Echinoderms, and their belonging to molluscs was discovered only through The Study of their internal anatomy.

Morphology. Mostly small worm-like organisms, ranging from 0.3 to 3 cm in length, with the largest reaching 30 cm. Their bilaterally symmetrical body is cylindrical, highly elongated, and lacks the typical mollusc division into HEAD, trunk, and FOOT (Fig. 209). On the ventral side, where the foot is located in some aplacophorans (subclass Solenogastres), There is a longitudinal furrow lined with ciliated epithelium (hence their

name - solenogasters). Within the furrow lies a narrow keel, also covered with ciliated epithelium, which represents a modified foot.

Fig. 209. Aplacophora (after Vlastov and Matyokin):

a - Chaetoderma nitidulum (subclass Caudofoveata); b - Neomenia carinata; c, d - Rhopalomenia aglaopheniae and Myzomenia sp. (subclass Solenogastres), respectively, on hydroid polyp colonies:

1 - gills; 2 - Mouth; 3 - ventral furrow

Integument. Externally, the body is covered with a cuticle bearing numerous calcareous spines and scales, which may be sharp or blunt.

The integumentary-muscular sac features well-developed musculature. Aplacophorans are more active animals than chitons; they crawl among the branches of coelenterate colonies or, less frequently, burrow into silt by bracing against the spines of their Skin.

The coelom of aplacophorans is a single cavity consisting of two parts: an anterior reproductive part and a posterior pericardial part with a single pair of coelomoducts that expel Gametes (Fig. 210).

Fig. 210. Diagram of the coelom Structure in Rhopalomenia acuminata (after Beklemishev):

1 - reproductive region of the coelom; 2 - Pericardium; 3 - seminal receptacle; 4 - cloaca; 5 - coelomoduct

Digestive System. The gut is short, appearing as a straight tube with a weakly developed radula, which is absent in some species. The Liver is poorly differentiated. Aplacophorans feed primarily on animal matter: species inhabiting hydroid and octocoral colonies graze on the polyps and coenosarc, whereas sediment-dwelling forms are mainly detritivores and microphages (feeding on foraminiferans, radiolarians, and small crustaceans).

Excretory Organs are absent; their function is performed by the intestine.

The Circulatory system has a very simple structure. Aplacophorans possess a Heart consisting of a single ventricle surrounded by a pericardium; atria are absent. True Blood Vessels are absent, and blood circulates through sinuses and lacunae.

Respiration in most aplacophorans occurs through the body surface.

In some species, skin folds form in the cloaca that function as gills, while members of the family Chaetodermatidae even possess a pair of small ctenidia.

The Nervous system is similar in structure to that of chitons; however, unlike the latter, solenogasters have well-developed cerebral ganglia (Fig. 211).

Fig. 211. Central Nervous System of aplacophorans (from Beklemishev):

1 - cerebral ganglia; 2 - pleurovisceral cords; 3 - pedal cords; 4 - posterior commissure of the pedal and pleurovisceral cords; 5 - pedal commissures; 6 - anterior pedal ganglia; 7 - sublingual ganglia; 8 - lateral ganglia

Sense Organs are absent, represented only by sensory papillae scattered across the body and isolated sensory Cells within the epidermal epithelium. Recently, a balancing organ—the statocyst—has been discovered in some species.

Reproductive System. Most aplacophorans are hermaphroditic, although dioecious species also occur, such as those of the genus Chaetoderma.

The Gonads are either paired or single and unpaired; the ducts (oviducts or sperm ducts) are always paired, though their terminal ends may fuse in some species. The ducts open into the cloaca at the posterior end of the body. Copulatory organs are absent.

Reproduction. Fertilization is external. Development proceeds with metamorphosis. The larva is a trochophore that grows uniformly, elongates, and gradually acquires a worm-like shape (Fig. 212).

Fig. 212. Metamorphosis of aplacophorans

(from Ivanova-Kazas): a - trochophore; b, c - later larval stages of Epimenia verrucosa; d - larva of Myromenia:

1 - prototroch; 2 - mouth; 3 - ventral furrow; 4 - telotroch; 5 - remnants of the episphere; 6 - calcareous scales

The class Aplacophora is divided into two subclasses: the more species-rich Caudofoveata and the Solenogastres. In the latter, the ventral furrow is well developed, whereas in Caudofoveata it is either absent or small, located at the posterior end of the body to form a pit that in some species contains a pair of ctenidia.

In northern seas at depths of 30-80 m, the species Chaetoderma nitidulum (Fig. 209) is widespread, reaching up to 8 cm in length; the deeper-dwelling Proneomenia sluiteri (14 cm long) and P. thulensis (2 cm long) are found at depths ranging from 100 to 150 m.