322 INVERTEBRATE MORPHOLOGY. 



a priori grounds seems improbable ; but there seems to be no good reason, 

 if the aquatic forms are derived directly from marine ancestors, why their 

 ctenidia should have become replaced by a lung, since in the aquatic Pro- 

 sobranch Paludina the ctenidium is still retained. On the other hand, it 

 may be again mentioned that the terrestrial Prosobraucbs such as Cyclo- 

 stoma, Acicula, etc., have lost their ctenidium and resemble a Pulmonate 

 in their mode of respiration. 



III. CLASS SCAPHOPODA. 



The class Scaphopoda contains a small number of closely- 

 related genera of marine Mollusca, Dentalium, Siplionodenta- 

 lium, Cadulus, etc., living imbedded in the sand in depths of 

 from 10 to 100 fathoms and possessing but slight powers of 

 locomotion. They resemble the Gasteropoda in possessing a 

 visceral hump which is relatively enormously elongated but 

 does not undergo a spiral twisting, nor has it fallen over to 

 the right or left side of the body. Consequently the Scaph- 

 opods are bilaterally symmetrical and stand in marked con- 

 trast in this respect to the Gasteropods. 



The mantle-folds are two in number, arising from the 

 anterior surface of the visceral hump and extending around 

 the body so as to completely enclose it, meeting posteriorly 

 and fusing together, except for a short extent, dorsally and 

 ventrally, and forming thus a tube to the anterior wall of 

 which the body is as it were attached. This tube is open at 

 either end, the ventral opening being somewhat larger than 

 the dorsal one, and the whole is enclosed within a tubular 

 shell (Fig. 145, sli) whose shape corresponds essentially to that 

 of the mantle. From the ventral opening the foot (/) pro- 

 jects to a greater or less extent, being in Dentalium a cylin- 

 drical structure, terminating in a conical process provided 

 with two lateral lobes. 



The mouth (ni) is situated at the extremity of a cylindrical 

 proboscis (not to be confounded with the protrusible proboscis 

 of a Gasteropod) and is surrounded by a number of leaflike 

 tentacles, while at the base of the snout there is upon each 

 side a bunch of long filamentous tentacles (t) capable of being 

 protruded from the mouth of the shell and of being withdrawn 

 within it. Each tentacle terminates in a spoon-shaped struct- 



