572 THE INVERTEBRATA 



lowed it is digested in a large crop by the action of the juices of the 

 digestive gland. 



The reduction of the shell is shown in the slugs, some of which, 

 like Testacella, have a small cap-like shell, which cannot possibly con- 

 tain the visceral hump, while others have an internal horny disc like 

 the shell of Aplysia and still others none at all. The mantle cavity of 

 slugs opens by a pneumostome but there are no respiratory move- 

 ments as in Helix. In other respects the organization of the slugs is 

 very similar to that of snails. 



The details of reproduction and development are uniform through- 

 out the group, but in some snails like Bulimus, the amount of albumen 

 added as food for the developing embryo is so great that the egg is 

 the size of a bantam's tgg. 



Class SCAPHOPODA 



Bilaterally symmetrical Mollusca with a tubular shell open at both 

 ends, a reduced foot used for burrowing, a head with many pre- 

 hensile processes, a radula, separate cerebral and pleural ganglia; 

 ctenidia absent and circulatory system rudimentary; and a trocho- 

 sphere larva. 



This is a small group of molluscs which in some ways stands be- 

 tween the Gasteropoda and the Lamellibranchiata. They are greatly 

 specialized for burrowing. Thus the shell is tubular and perforated 

 at the apex. The foot emerges from the wider opening, while the apex 

 remains above the surface of the sand when the animal is burrowing, 

 and serves alike for the entrance of water into and its exit from the 

 mantle cavity. The head is proboscis-like in form and has none of the 

 usual sense organs, but in Dentalium (Fig. 391), the one common 

 genus, there are extensible filaments, the captacula, with sucker-like 

 ends, which arise from the dorsal side of the head and serve partly 

 as sense organs and partly for seizing the food. The foot is conical 

 and can be protruded for use as a digging organ. 



There is a well-developed radula, a mantle, which in the larva is 

 produced into two lobes (which fuse later), a nervous system with 

 separate cerebral and pleural ganglia and a symmetrical visceral loop. 

 The kidneys are paired; they do not have an opening into the peri- 

 visceral coelom. These characters, with the exception of the first and 

 last, bring the Scaphopoda near to the primitive lamellibranch. In 

 the two following morphological features the group is so specialized 

 that it stands apart from any other division of the Mollusca. 



There are no ctenidia, respiration taking place by means of the 

 m?ntle. The circulatory system is remarkably simplified and there is 

 no distinct heart. 



