100 GASTROPODA. 



attached for its whole length (save in Valvafa). The 

 nerve-ganglia distinct and concentrated round oesophagus ; 

 pedal commissures rare. Genital aperture distinct, 

 dioecious with rare exceptions. 



To this order belong by far the greater number of 

 Prosobranchia, all, indeed, of those the development of 

 which is dealt with here except the forms named above. 

 Janthina, Murex, Baccinnm, Purpura, Nassa, Fulgur, 

 Fusux, Fasciolaria, Strombus, Rostellaria, Crepidula, 

 Calyptraea, Vrrmetus, Bythinia, Paludinq, Thyca, Stili- 

 fer, Entoconcha, etc. 



Sub-order 3. — Heteropoda (Nucleobranchia). The char- 

 acter of the nervous system, the position of the gill, 

 ventricle and auricle the same as in the Monotocardia. 

 Foot developed into a fin. 



Oxygyrus, Af/m/ta, Pfemtrachea, Carinaria, Firoloida. 



II. Opisthobranchia. 



The gill and auricle generally behind the ventricle (except in 

 Actaeon). Pleurovisceral commissures rarely crossed 

 {Actaeonvlae). Hermaphrodite, marine. 



Sub-order 1. — Tectibranchia. Shell generally present, often 

 much reduced and interna], wanting in Rancina and 

 Pleurobranchea ; with mantle-cavity containing a cteni- 

 dium. 



Actaeon, Bulla, Acera, Gasteropteron, Philine, Apli/sia, 

 Pleurobranchun, Pleurobranchea, Umbrella. 



Sub-order 2. — Nudibranchia. Without shell in adult stage ; 

 mantle, ctenidium and osphradium wanting. 



Tritonia, Do?'is, Chromodon's, Polycera, Tergipes, Elysia, 

 Aeolin, Doto, Fiona. 



III. Pteropoda. 



Pelagic Gastropods in which the head is much reduced, and 

 the foot is developed like a fin ; now generally classed 

 with the Opisthobranchia. 



Sub-order 1. — Thecosomata. With calcareous or carti- 

 laginous shell, with mantle and mantle-cavity. 



Spirialis, Liwacina, Tiedentannia, Cymbub'a, Cavolinia, 

 Hyalocylix, Styliola, Cleodora, Creseis. 



