CLASSIFICATION OF GASTEROPODA. 357 



The study of development may be more profitably followed 

 in the pond snail Limnceiis, where gastrula, trochosphere, and 

 veliger can be readily seen. 



Classification of Gasteropoda. 

 Sub-Class I. Streptoneura. 



(i) Zygobranchia, e.g., limpet. 

 (2) Azygobranchia, e.g., whelk. 



Sub-Class II. Euthyneura. 



(1) Opisthobranchia, e.g., Aplysia. 



(2) Pulmonata, e.g.. Snail. 



I. Streptoneura (i.e., with twisted nerves). In this division the 

 torsion of the body has twisted the visceral nerve loop in the 

 form of a figure 8. The sexes are separate, 

 (i) Zygobranchs. Both gills may persist, or both may 

 degenerate, their functions being then discharged by 

 folds of the mantle. Both nephridia persist, that on 

 the right side being the larger, and serving also as a 

 genital duct. 

 Examples — 



Limpet (Patella), Ear shell (Haliotis). 

 (2) Azygobranchs. The morphologically left gill, nephridium, 

 and osphradium disappear, those morphologically right 

 persist topographically on the left. It may also be 

 that the genital duct is a modification of the morpho- 

 logically left nephridium. 

 Examples — 



Periwinkle (Littorina), Buckie (Buccinuni), Dog 

 whelk (Purpura), Cone shells (Coiius), Murex, 

 lanthiiia ; and also the pelagic Heteropods, with 

 foot adapted for swimming, e.g. , Atlanta (large shell), 

 Carinaria (small shell), Pterotrachea (no shell). 



II. Euthyneura (i.e., with straight nerves). The torsion of the 

 visceral hump is less complete, and the visceral loop is 

 untwisted. All are hermaphrodite. The shell is often 

 light, and may be absent from the adult. The mantle may 

 also be much reduced. The gill (if present) is behind the 

 heart (Opisthobranch condition), whereas in the Strepto- 

 neura it was almost always in front (Prosobranch condition). 

 (i) Opisthobranchia. Small visceral hump, rarely any shell 



in the adult ; anus behind, and heart in front of gill 



when this is present. 



(a) Tectibranchia. Well developed mantle fold and 



functional gill, usually with delicate shell. 

 Bulla, Aplysia, Dolabella, Umbrella. 



(b) Nudibranchia. Mantle fold rudimentary and shell 



absent in the adult. The gill is either much modified 

 or absent. 



Triton, Doris, Eolis. 



