;78 



MOLL use A. 



in tlie definitive condition tlie velum is nsually develoiDed in 

 embryonic life, sometimes so strongly that the embryo rotates in 

 the surrounding fluid. 



Order I. Prosobranchia. 



The Prosobranchs, lilce most gasteropods, have the twisting of 

 the visceral com})lex from left posterior to right anterior, so tliat 

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

 missures are twisted into an S, and the iiephridia 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 sijihonostomate or holostomate (p. 371). Certain 

 Prosobranchs are near the primitive Amjjhineura in the reten- 

 tion of both ctenidia, both auricles, and both nephridia, but in the 

 great majority only one gill (the primitive right) is jDresent and the 

 corresponding auricle alone is well developed, although the other 

 may exist in a rudimentary condition. 



Sub Order I. ASPIDOBRANCHIA (Diotocardia, Scutibranchia). 

 Ctenidium bipeotinate (fig. 373) or absent. There are usually two 

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



Fifi. 37a. Fio. :)r3. 



Fig. .372. — F^issMrcffct padigoiiica, ventral view. (From Broun.) /»■, the paired gills; 



)), foot. 

 Fio. 373. — Acm(jio testudinalia, limpet. (From Binnuy-Uoiild.) 



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

 AcMTEiD^ with ctenidium. Acma'a * (fig. 373). Patellid.e, ctenidia 

 lacking, replaced by a ring-like mantle gill. Patella (fig. 363, ^4). ZYGO- 



