SCAPHOPODA. A group of marine invertebrate animals popularly called elephant's tusk shells and constituting a class of the Mollusca (q.v.). They are repre sented by 12 genera (of which Dentalium is the most familiar), and over 30o species, and are the smallest molluscan class. Their structure is quite distinct from that of other molluscs. They are bilaterally symmetrical, elongate animals in which the right and left edges of the mantle are joined in the mid-ventral line except at the anterior and posterior ends. The vis ceral mass is thus enclosed in a tubular sheath open at both ends. The shell se creted by the mantle is correspondingly tubular and with anterior and posterior orifices. The head is imperfectly developed and bears numerous long filaments. The foot is cylindrical and the animal is devoid of gills.
The Scaphopoda are exclusively marine.
The foot is adapted for digging and bur rowing into sand, in which they lie with the posterior extremity of the shell projecting from the surface.
They were originally placed in the same class (Acephala) as the Lamellibranchia ; but beyond the conformation of the mantle and the digging foot there is no close resemblance to that group, whereas their possession of a radula, mandible and buccal bulb and of a stomatogastric system in their nervous organization point to affinities with the Gas tropoda.
External Form.—The shell of Dentalium is able to contain the whole animal and is elongate, conical and slightly curved. There are two apertures in all the Scaphopod shells, a larger an terior one from which the foot projects and a smaller posterior one. The hinder end bearing this orifice is kept clear of the sand and thus admits water for respi ration, and allows the excreta and faeces to be discharged when the animal is buried. The mantle cavity is continuous from one end of the body to the other. The head and foot lie at the anterior end, the former above the latter. The head is cylindrical and bears two lobes beset with long fila ments (captacula). These are mainly sensory but also serve to capture the small organisms on which the animal feeds. The foot is elongated and capable of con siderable extension. Its expansi ble end is of great service in digging.
Internal Anatomy.—All Scaphopoda have a well-devel oped buccal mass with mandible and radula. From the oesophagus the food passes into the stomach, which is little differentiated and receives the ducts of a bilobed liver and a pyloric caecum. The
intestine is provided with an anal gland. The circulation and res piratory system is extremely simple. The heart is rudimentary and there are no proper blood vessels. There are no specialized respiratory organs (gills) and the blood is oxygenated in the inner surface of the mantle. There are two kidneys in the mid-ventral region of the body which open to the exterior, one on each side of the anus. The nervous system consists of the same pairs of ganglia with their commissures as in the Gastropoda. The cerebral and pleural ganglia are joined to the pedal ganglia by a long connective. These ani mals have only three kinds of sensory or gans—the captacula, apparently tactile and olfactory, the subradular organ, prob ably an organ of taste ; and the organs of balance (otocysts), situated in the foot. The sexes are separate. The ovary and the testis are unpaired and open into the right kidney, as in the aspidobranchiate Gastro poda.
Development.—This has been studied in Dentalium. The eggs are laid singly and segmentation is unequal and irregular. A gastrula arises through invagination, as in most Mollusca, and subsequently devel ops into a floating trochophore larva. A veliger stage succeeds and after five or six days the velum (girdle of cilia) atrophies and the young Dentalium abandons its floating life and starts to creep about on the sea-bottom. Interesting experiments have been done on this form. (See EXPERIMENTAL EMBRYOLOGY.) Bionomics, Evolution, Etc. —The Scaphopoda are sedentary and live in the adult stage on the sea-bottom, into the surface lay ers of which they burrow and usually remain with part of the shell projecting from the surface. They are carnivorous and feed upon such small animals as Fora menifera, young bivalves, etc. The majority live in fairly deep water and (e.g.) in the North sea are entirely absent in the lit toral zone. Dentalium peruvi anum has been found at a depth of 2,235 fathoms (U.S.S. "Alba tross"). They have a practically cosmopolitan distribution. The earliest representatives of the class appear in the Middle Silurian, some 285 fossil species being known.
BIBLIOGRAPHY.—The most important papers up to 1906 are cited by P. Pelseneer in Lankester's Treatise on Zoology (Pt. v. Mollusca). See also:—T. van Benthem Jutting, "Die Tierwelt der Nord-und Ostsee" IX. c. 1926 ; J. Henderson, "A Monograph of the North American Scaphopod Molluscs," United States National Museum, Bull.