INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA

PHYLUM MOLLUSCA

CLASS MONOPLACOPHORA

Monoplacophorans were long known only from the fossil record in Cambrian, Silurian, and Devonian marine deposits (about 185 species described). It was not until 1952 that several specimens of living monoplacophorans were discovered in the Peru-Chile Trench at a depth of 3570 m, which

were described under the name Neopilina galatheae in honor of the expedition vessel. Currently, 14 extant species of monoplacophorans are known.

Anatomy and Morphology. Monoplacophorans possess a single, cap-shaped, saucer-shaped, or spirally coiled shell (Fig. 229). The shell of Neopilina (about 3 cm in diameter) is cap-shaped with a circular lower margin and an apex shifted forward, bearing a tiny apex whorl. The body is bilaterally symmetrical, consisting of a small HEAD, a relatively high trunk, and a disc-shaped FOOT. The head is located on the ventral side of the body and is barely demarcated from the trunk. The Mouth is situated on the head, preceded by a pair of tentacles and a specialized cutaneous fold called the velum, near the ends of which, posterior to the mouth, lies a pair of bush-like tentacles. The integument of the lower margin of the trunk transitions into a circular cutaneous fold—the mantle—which surrounds the head and foot and aligns with the edge of the shell. A broad mantle furrow runs between the mantle, head, and foot; five to six pairs of gills and six pairs of excretory openings are located within this furrow on either side of the foot. The anus is situated at the posterior end of the body behind the foot (Fig. 229).

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Fig. 229. The monoplacophoran Neopilina galatheae (adapted from Dogel):

a, b - dorsal and ventral views, respectively:

1 - edge of the shell; 2 - mouth; 3 - velum; 4 - ctenidium; 5 - mantle furrow;

6 - edge of the mantle; 7 - anus; 8 - foot; 9 - chemoreceptor organ; 10 — head

The musculature consists of circular, oblique, and longitudinal Muscles of the foot and mantle; in addition, there are eight pairs of retractor muscles extending from the foot to the DORSAL SIDE OF the shell, where they insert. These muscles exhibit a metameric arrangement and permeate the entire body of the mollusk.

Body Cavity. The coelom consists of the Pericardium surrounding The Heart, and two large dorsal coelomic sacs. There are six pairs of celomoducts, two of which originate from the pericardium, while the remaining ones arise from the dorsal coelomic sacs (Fig. 230). All of them open into the mantle furrow near the Base of the gills and function as excretory Organs, or Kidneys.

Fig. 230. Diagram of the anatomy of Neopilina (adapted from Kilias, with modifications):

1 - cephalic tentacle; 2 — foot muscles; 3 - kidneys; 4 - excretory opening; 5 - ctenidium; 6 - ventricle of the heart;

7 - anus; 8 - atrium; 9 - gonad; 10 - duct connecting the Kidney to the coelom;

11 - pleurovisceral nerve cord; 12 - velum

Digestive System. The Pharynx contains a well-developed radula equipped with numerous chitinous Teeth. A crystalline style is present in The Stomach.

The digestive gland (Liver) is well-developed and consists of two symmetrical lobes. The midgut is long, forms several loops, and transitions into a broad hindgut that terminates in the anus at the posterior end of the body.

Circulatory system. The heart lies within the pericardium and consists of two ventricles and four atria. The anterior tapered ends of the ventricles merge to form an aorta, which delivers Blood to the anterior end of the body. From the aorta, blood empties into a system of sinuses surrounding the gut, digestive gland, Gonads, and other visceral organs, supplying them with oxygen and nutrients. Venous blood from the body lacunae flows into the gills. Oxygenated blood from the gill lacunae and sinuses enters the atria, with blood from the posterior pair of gills flowing directly into the posterior pair of atria, whereas blood from the other gills enters longitudinal venous sinuses that transport it to the anterior pair of atria. From the atria, blood is pumped into the ventricles.

The respiratory organs are paired gills, or ctenidia, located in the pallial groove on either side of the foot. They are pectinate in shape rather than plumelike, as seen in most Mollusks. Neopilina galatheae possesses five pairs of ctenidia, Vema ewingii has six pairs, and Micropilina has three.

The Nervous system is similar to that of chitons, but unlike the latter, it features well-defined cerebral ganglia connected by a long commissure. The cerebral ganglia are linked to two pairs of nerve cords: a pedal pair and a pleurovisceral pair. The pedal cords have only two commissures: one anterior and one posterior, which connects them just in front of the anus. The pleurovisceral cords join each other at the posterior end—also anterior to the anus—and connect with the pedal cords via ten pairs of commissures.

Sense Organs are poorly developed, and eyes are absent. There is a pair of short cephalic tentacles and a pair of branching post-oral appendages that function as chemoreceptors. In addition, a pair of statocysts (organs of equilibrium) is located behind the head.

Reproductive System. Monoplacophorans are dioecious. They possess two pairs of lobed gonads situated outside the coelom, ventral to the intestine. They lack dedicated gonoducts; instead, their ducts connect with the third and fourth pairs of kidneys, through which Gametes are released into the environment. Copulatory organs are absent, and Fertilization is external. The embryonic development of monoplacophorans remains unstudied.

It is also unknown whether they possess a larval stage.

Data on the lifestyle of monoplacophorans are scarce. They are inhabitants of the deep ocean. Neopilina galatheae lives on muddy bottoms at depths ranging from 2,500 to 3,600 m, while Vema occurs at depths up to 5,000 m. They feed on detritus gathered from The surface of bottom sediments. Their gut contents have been found to contain diatoms, foraminiferans, and giant xenophyophore rhizopods.