CHAPTER XXXII. 



GASTROPODA. 



Systematic Order : — 



1. Prosobranchia (Streptoneura). 



The gill or gills lie in front of the heart. The pleurovisceral 

 connectives are crossed. The sexes are distinct (save in 

 Valvata, Marsenina and a few parasitic forms). 



Sub-order 1.- — Diotocardia. The heart has usually two 

 auricles. The ctenidia are bipectinate and free distally. 

 The pedal centres form long ganglionic cords connected 

 by transverse commissures and closely associated with 

 the pleural centres. Gonad opens into right nephridium 

 (save in Neritidae). The nephridium is generally paired. 



(") Zygobranchia. Ctenidium paired, ventricle tra- 

 versed by rectum, two nephridia ; shell with apical or 

 marginal slit or row of perforations. 



Haliotis, FusmreUa, Pleurotomaria. 



(/') A zygobranchia. One ctenidium (left of Zygo- 

 branchs) ; two auricles (right ending blindly) ; heart 

 traversed by rectum (except in Helicinidae) ; nephridium 

 generally paired, operculate. 



Turbo, Trnchus, Neritina (one kidney, distinct genital 

 aperture). Helicina (pulmonate, no ctenidium, one 

 auricle). 



(c) Docoglossa. Gill single or absent ; heart with 

 single auricle, ventricle not traversed by rectum ; two 

 osphradia : two kidneys. 



Patella (ctenidia absent), Acmaea. 



Sub-order 2. — Monotocardia. Heart with one auricle ; 

 kidney and gill unpaired, the latter monopectinate and 



