III. SCAPHOPODA. IV. GASTEROPODA. 



Class III. Scaphopoda (Solenoconchae). 



The tooth shells are primitive forms which have some resem- 

 blances to the Acephala in the paired 

 liver and iiephridia and in structure of the 

 nervous system (with the exception that a 

 buccal ganglion is present and the pleural 

 ganglia are distinct from the cerebral). 

 In some points they are primitive (persis- 

 tence of jaws and radula), but in others 

 they are considerably modified. They lack 

 gills, have unpaired dioecious gonads, 

 rudimentary heart (no auricle), and have 

 two bunches of thread-like tentacles either 

 side of the mouth. The mantle lobes, 

 which are paired in the larva, unite below, 

 forming a sac open at either end, and this 

 secretes a shell shaped like the tusk of an 

 elephant, from the larger end of which 

 protrudes the long three-lobed foot used 

 for boring in the sand. Dentalium (fig. 361), Entalis* 



FIG. 361. Dentalium elephan- 

 tinum, tooth shell ; left the 

 animal, right the shell 



foot ; ?, liver region; o, 

 inder opening of mantle. 



Class IV. Gasteropoda. 



Although more highly organized than the Acephala, the snails 

 are in some respects more primitive. The regions of the body- 

 foot, visceral sac, head, and mantle occur in all orders, although 

 in each one or more forms may occur in which one or another part 

 is lost. 



As a rule the foot is flattened ventrally to a creeping sole. In 

 it may be distinguished anterior and posterior processes, the pro- 

 podium and metapodium, a sharp lateral margin, the parapodium, 

 and, above these, appendages or ridges, the epipodia. Inside the 

 foot is usually a pedal gland. 



The head bears (1) the tentacles, a pair of muscular lobes or 

 hollow retractile processes; (2) a pair of primitive vesicular eyes, 

 which usually lie at the basis of the tentacles, but may rise even 

 to their tips. In many snails the eyes are on special stalks which, 

 as in the stylommatophorous Pulmonata, form a second pair of 

 tentacles. The protrusion of the tentacles is caused by an inflow 

 of blood, their retraction by muscles attached to the tip which 

 draw them in like a finger of a glove. 



