SCAPHOPODA. 



819 



Class IV. Scaphopoda. 



This class of Molluscs includes only the single family of the 

 Dentaliida, the characters of which would assign to it a position in 

 some respects intermediate between the Lamellibranchs and the 

 Gastropods. The type of the group is the genus Dentalium, the 

 animal of which is bilaterally symmetrical, and is enclosed in a con- 

 tinuous mantle, which secretes a tubular shell, which is open at both 

 ends. From the large anterior aperture of the shell is protruded the 

 foot, with a circle of tentacles surrounding the mouth, but there is 

 no distinct head. The oral tentacles appear to discharge a respi- 

 ratory function, but there are no specialised branchiae, nor is a 

 definite heart present. The nervous system is of the Molluscan 

 type ; the kidneys are paired ; the sexes are distinct ; and the 

 pharynx is furnished with a radula. 



The shell in the Dentaliida is in the form of a slightly curved 

 calcareous tube (fig. 735), of conical form, but not spirally coiled. 

 The shell is open at both 

 ends, the anterior aperture 

 being larger than the pos- 

 terior, and the latter being 

 sometimes simple, some- 

 times crenulated or fissured. 

 The concave side of the tube 

 corresponds with the dorsal 

 side of the animal, and the 

 ventral side is convex. No 

 operculum is developed. 



The Dentaliidce are all 

 marine, and live buried in 

 sand or mud, with the wide 

 anterior extremity of the 

 shell downwards. The fos- 

 sil forms begin at least as 

 early as the Devonian, and 

 some Ordovician and Si- 

 lurian types have been re- 

 corded, though these are 

 not free from doubt. 



In the genus Dentalium 

 (figs. 734 and 735) the tubular shell is smooth, or longitudinally 

 striated, or annulated ; the anterior aperture is simple and is not 

 contracted ; and the posterior aperture is typically truncated and 

 entire. Entalis is hardly separable from Dentalium proper, but there 

 is a short slit at the posterior aperture on the ventral (convex) side 



Fig. 734. — Dentalium vul- 

 gare, of the natural size, with 

 the oral tentacles protruded from 

 the anterior opening of the shell. 

 (After Lacaze-Duthiers.) 



Fig- TiS-— Den- 

 t alitim B oue i. 

 Miocene. (After 

 Deshayes.) 



