SCAPHOPODA, 



405 



Class III. SCAPHOPODA. 



Very different in many respects from Gasteropoda are the Scaphopoda, 

 of which Dentalimn (Elephant's tooth-shell) is the commonest genus. 

 They are apparently related to the Zeugobranchiate Gasteropods, and 

 also to the simplest Bivalves. They burrow in the sand at consider- 

 able depth off the coasts of many 

 countries. The mantle has origin- 

 ally two folds, which fuse ventrally, 

 and the shell becomes cylindrical, 

 like an elephant's tusk. It is open 

 at both ends. The larger opening 

 (directed downwards in the sand) is 

 anterior, the concave side of the 

 shell is dorsal. The mouth opens 

 at the end of a short buccal tube, 

 at the base of which is a circle of 

 ciliated tentacles. The foot is long, 

 with three small terminal lobes. It 

 is used in slow creeping, and is pro- 

 truded at the anterior opening. 

 There are cerebral and pleural 

 ganglia near one another in the 

 head, pedal ganglia in the foot, and 

 a long untwisted visceral loop with 

 olfactory ganglia near the posterior 

 anus. Sense organs are represented 

 by otocysts beside the pedal ganglia. 

 There is an odontophore with a 

 simple radula. The food consists 

 of minute animals. There is no 

 heart, but colourless blood circulates 

 in the body cavity. There are two 

 nephridial apertures, one on each 

 side of the anus ; and two neph- 

 ridia. The sexes are separate ; the 

 reproductive organ is simple and 

 dorsal in position ; the elements 

 pass out by the right nephridium. 

 The gastrula is succeeded by a free- 

 swimming stage, in which there is a 

 hint of a velum and a rudimentary 

 shell 



FIG. 202. Proneomenia. 

 vous system. From 

 recht. 



Ner- 

 Hub- 



Examples. Dentaliuiii, Ental- 

 htm. About forty widely- 

 distributed species are known. 



.g-., Cerebral ganglia ; s/g: sub- 

 lingual ; a.p.g-., anterior pedal ; 



P-P-g-i posterior pedal ; p.v.g., 

 posterior viscerals ; si., sublingual 

 connectives ; cpc., cerebro-pedal 

 connective ; pe., longitudinal pedal 

 nerves ; /., longitudinal lateral 

 nerves. 



Dentalium entale occurs off 



British coasts. The genus occurs as a fossil from Carboniferous 

 (or perhaps earlier) strata omvard. 



