378 MOLLUSC A. 



in the definitive condition the velum is usually developed in 

 embryonic life, sometimes so strongly that the embryo rotates in 

 the surrounding fluid. 



Order I. Prosobranchia. 



The Prosobranchs, like most gasteropods, have the twisting of 

 the visceral complex from left posterior to right anterior, so that 

 the anus lies on the right side near the head, the nervous com- 

 missures are twisted into an 8, and the nephridia of the right 

 side have been carried to the left, where they lie far forward. 

 This has twisted the heart so that it receives branchial blood from 

 in front and sends it backwards through the aorta. The sexes 

 are separate and the shell and mantle are usually well developed. 

 Accordingly as the mantle is drawn out in a siphon or not, the 

 shells are siphonostomate or holostomate (p. 371). Certain 

 Prosobranchs are near the primitive Amphineura in the reten- 

 tion 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. 



Sub Order I. ASPIDOBRANCHIA (Diotocardia, Scutibranchia), 

 Ctenidium bipectinate (fig. 372) or absent. There are usually two 

 auricles and two nephridia. DOCOGLOSSA (limpets), auricle single ; 



FIG. 372. FIG. 373. 



FIG. 372. Fissurella patagonica, ventral view. (From Bronn.) br, the paired gills; 



p, foot. 

 Fio. 373. Acmcea testudinalis* limpet. (From Binney-Gould.) 



one or no ctenidinm ; intestine not passing through heart, shell conical. 

 ACM.EIDJE with ctenidium. Acmcza* (fig. 373). PATELLID.E, ctenidia 

 lacking, replaced by a ring-like mantle gill. Patella (fig. 362, A). ZYGO- 



