IV. GASTEROPODA; PROSOBRANCHIA 333 



In the development the great constancy with which the veliger stage 



(figs. 313, 314) appears is noticeable. Most marine Iarv£c swim at the 



surface by their velum before creeping at the bottom. But in those cases 



where the snail leaves the egg in the adult form the velum is usually 



developed in embryonic life, sometimes so strongly that the embryo 



rotates in the surrounding ihiid. 



« 



Order I. Prosobranchia. 



In the prosobranchs the twisting of the body has brought the ctenidia 

 and consequently the auricles in front of the ventricle, and hence the aorta 

 leads backwards. It also results in the twisting of the nervous system 

 like the figure 8. The sexes are separate and the shell and mantle are 

 usually well developed. Certain Prosobranchs are near the primitive 

 Amphineura in the retention of both ctenidia, both auricles, and both 

 nephridia, but in the great majority only one gill (the primitive right) is 

 present and the corresponding auricle alone is well developed, although 

 the other may exist in a rudimentary condition. 





z 





Fig. 340. — Fissurella palai^onira, Fig. 341. — Actiiwa Ic^liidinalis* 'lira-pet 



ventral view (from Bronn). br, the (from Ijinncj'-Gould). 



paired gills; p, foot. 



Sub Order I. ASPIDOBRANCHIA (Diotocardia), Ctenidia bipectinate 

 (fig. 340) or absent. Usually two auricles and two nephridia. DOCOGLOSSA 

 (limpets), auricle single; intestine not passing through heart, shell conical. 

 AcM^iD.5E with ctenidium. Acnicca* (fig. 341). Patellid.!, ctenidia replaced 

 by a ring-like mantle gill. ZYGOBRx\NCHI.\. Two ctenidia (fig. 340), shell 

 with marginal slit or with holes; auricles and usually nephridia paired; heart 

 traversed by intestine. Fissurellid.5;, keyhole limpets; shell conical, with apical 

 opening. Haliotid.e, abalones (fig. 330, B). AZYGOBR.\NCHIA. One 

 ctenidium, two auricles. Trochid.e, Trodms, Margarita* Turbinid.e, top 

 shells; Sub Order II. PECTINIBRANCHIA (Monotocardia). Ctenidium uni- 

 pectinate, a single auricle, osphradium well differentiated (fig. ^^^t,), intestine 

 not passing through the heart. Many groups based upon structure of lingual rib- 



