INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA

PHYLUM MOLLUSCS (MOLLUSCA)

CLASS TUSK SHELLS (SCAPHOPODA)

This Class comprises a small group of marine benthic Molluscs distributed throughout the World Ocean, particularly in subtropical and tropical zones, from the littoral down to depths of 7,000 m. About 300 extant species are known; one species, Dentalium novemcostatum, inhabits the Black Sea.

Scaphopods are predominantly small molluscs ranging from 3 mm to 13 cm in length; the largest of them is the elephant tusk shell (Dentalium elephantium), which reaches 15 cm. The elongated, narrow body of scaphopods is entirely enclosed in a continuous tubular shell open at both ends (Fig. 227). The HEAD and FOOT protrude from the wider anterior opening, while only a small appendage of the mantle margin extends from the posterior end. The shell is slightly curved, resembling an elephant's tusk.

Fig. 227. Diagram of the anatomy of Dentalium (after Dogel):

1 - tentacles; 2 - Mouth; 3 - cerebral ganglion; 4 - Stomach; 5 - Pericardium with Heart; 6 - Kidney;

7 - gonad; 8 - upper mantle aperture; 9 - digestive gland (Liver); 10 - anus; 11 - shell; 12 - mantle;

13 - statocyst; 14 - pedal ganglion; 15 — foot

The shell consists of two layers: the prismatic and the nacreous (pearly) layers. The horny layer is undeveloped, which is why the shell is typically white.

The small head appears as an outgrowth bearing the mouth at its tip, along with two bundles of numerous long, slender tentacles that are thickened at their ends and function in food gathering and as Organs of Touch. Glandular Cells are located on the spoon-shaped expansions of the tentacles and secrete mucus, causing small organisms—the food of scaphopods—to adhere to them. The foot is cylindrical, terminating in a conical shape with a pair of lobes, or it is elongated and worm-like, ending in a denticulated disc with a thread-like process at its center. It is well-adapted for burrowing in sediment, which explains the Ukrainian common name for the class, "lopatonohi" (shovel-footed). During locomotion, the foot extends strongly, the lateral lobes fold inward, and it easily penetrates the substrate. Then, the lateral lobes (or disc) expand and, acting like an anchor, secure the end of the foot in the sediment. As the foot contracts, the mollusc's body is pulled forward.

The mantle enveloping the visceral mass fuses around it into a continuous tube. The widened anterior part of the mantle cavity contains the anus and renal openings, whereas the posterior part is narrow and terminates in an opening at the tip of the shell. Water is drawn in through the posterior opening and, after supplying oxygen to the Tissues, is expelled through the same opening, carrying away faeces, excretory products, and, during the spawning season, reproductive products. Water Circulation within the mantle cavity is maintained by the beating of the ciliated epithelium of the mantle.

Digestive System. The mouth leads into a Pharynx equipped with a horseshoe-shaped jaw and a radula; an Esophagus, stomach, bilobed liver, and mid- and hindgut are present. The intestine forms a loop (anopedal flexure); the anus is located near the anterior end of the body. Scaphopods feed on small benthic invertebrates such as foraminiferans, ostracods, and juvenile bivalves, extracting them from the sediment using their adhesive tentacles.

The excretory organs consist of a pair of Kidneys that do not communicate with the pericardium but open solely into the mantle cavity.

The Circulatory system is reduced. Blood Vessels are absent, but There is a heart surrounded by a pericardium, consisting of a single ventricle; blood flows exclusively through lacunae and sinuses.

Respiratory organs. Ctenidia are absent, and Respiration is carried out via thin folds of the mantle located on the walls of the mantle cavity.

The Nervous system consists of four pairs of ganglia connected by connectives. The paired cerebral and pleural ganglia lie closely appressed to one another, while the pedal and visceral ganglia are located in the foot and near the anus, respectively. Buccal ganglia innervating the pharynx are also present.

Sense Organs are poorly developed. Eyes are absent; there is a pair of equilibrium organs, the statocysts. The oral tentacles function as organs of touch; The Organ of chemical sense, the subradular organ, is located in the pharynx.

Reproductive System. Scaphopods are dioecious. The gonad is unpaired, lacks its own genital ducts, and opens into the right kidney, through whose efferent duct the mature reproductive products are discharged.

Reproduction. Fertilization is external. Embryonic development follows the pattern typical of molluscs: a trochophore larva hatches from the egg. Later, a shell anlage forms on the DORSAL SIDE OF the larva as a thin plate that folds over the dorsal region, while its free margins fuse ventrally into a tube. A disc-shaped velum forms on the upper hemisphere of the larva; such a larva is called a veliger (Fig. 228). After several days of planktonic life, metamorphosis begins: the velum degenerates, and the larva settles to the bottom.

Fig. 228. Development of Dentalium (from Vlastov and Matyokin):

a, b — early and late veligers, respectively: 1 — velum; 2 — mantle; 3 — shell primordium; 4 — foot primordium

Scaphopods mainly inhabit shallow waters, living in soft sediments and burrowing in such a way that only the posterior end of the shell protrudes above the surface.