III. SCAPHOPODA. IV. GASTEROPODA 



325 



Saxicavid^ have burrowing species. These forms connect with others 

 ( Teredo'*) in wliich the united siphons far exceed the rest of the body in length, 

 giving the animal a worm-like appearance (tig. 328). Since the valves do not 

 cover the whole body, they are supplemented by accessory shells, or the worm- 

 like body secretes a tube in which the rudimentary valves are imbedded. 

 The Pholadid^ (phosphorescent), burrow in wood, clay, or stone. The 

 shell is well developed. In the ship worms (Teredid^) the shells, on the 

 other hand, are small, while some species line their burrows, with lime. The 

 species of Teredo* by their boring habits do much 

 damage to wood in the sea, especially in the tropics. 



Lastly, there should be mentioned the little-known 

 SEPTIBRANCHIATA, in which the gills have the 

 shape of a septum perforated by gill slits separating the 

 branchial and cloacal chambers. Silenia, Cuspidaria. 



Class III. Scaphopoda (Solenoconchse). 



The tooth shells resemble the Acephala in the 

 paired liver and nephridia and in the nervous system 

 (except that a buccal ganglion is present and the 

 pleural ganglia are distinct from the cerebral). In 

 some points they are primitive (persistence of jaws 

 and radula), but in others they are considerably 

 modified. They lack gills, have unpaired dicecious 

 gonads, rudimentary heart (no auricle), and have two 

 bunches of thread-like tentacles either side of the 

 mouth. The mantle lobes, 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. Dcntalium 

 (fig. 329), Entalis* 



Fig. 329. — Dentalium 

 elephantinum, tooth shell; 

 left ttie animal, right tlie 

 shell. /, foot; /, liver re- 

 gion; 0, hinder opening 

 of mantle. 



Class rV. 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 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 propodium and metapodium, a sharp lateral margin, 

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

 Inside the foot is usually a pedal gland. 



The head bears (i) 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 w^hich (stylommatophorous Pulmon- 

 ata) form a second pair of tentacles. The protrusion of the hollow 

 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. 



