MOLLUSCA 



261 



Class II. Gasteropoda {belly-footed; Snails, Slugs, Whelks, and Periwinkles). — 

 Gasteropods are moUusks with unsymmetrical, univalved, usually spiral shells 

 (occasionally lacking the shell altogether). The head and foot ordinarily preserve 

 the bilateral symmetry, but the other organs lose their symmetry both from the 

 spiral form of the shell and from a twisting which many of the forms undergo by 

 which the nervous system and certain other visceral organs lose their original right 

 and left relations. The head region is well developed, having tentacles, eyes, and 

 a mouth with a tooth-bearing radula. Gills in the mantle cavity two, one, or 

 none; in the air-breathing forms there may be merely a pulmonary sac. The sexes 

 are separate {Streploneura) or united in one individual (land snails). Development 

 is mostly indirect. 



Pig. 120. 



Pig. 121. 



Fig. 130. 



Acmaa lesludinalis (Limpet), from Binney's .Gould, 

 figure, dorsal view. 



Upper figure lateral view; lower 



Questions on the figure. — How do the Limpets differ from the majority of the 

 snails? What is the appropriateness of the specific name (testudinalis)? 



Fig. 121. Helix albotabris, a pulmonate Gasteropod. From Binney's Gould. 



Questions on the figure. — What is the significance of Helix? Of albolabris? 

 Identify the parts of the shell. Is it a right or left spiral? What do you mean by 

 your answer? 



Subclass I. Streploneura. — Gasteropods in which the nerve loop made by the 

 visceral commissures, is twisted in development into the form of the figure 8; the 

 other visceral organs are twisted so that right and left are interchanged. Only one 

 pair of tentacles on the head. Sexes separate. Gills usually in front of the 

 heart. 



One of the common representatives of this group is Littorina, the common 

 periwinkle of the seashore. Many other types of almost infinite variety of form, 

 size, and color inhabit the ocean, their shells often being washed ashore by the 

 waves; such are the cowries, the whelks, the cone-shells, etc. Here belong the 

 uncoiled Limpet and the slightly coiled Crepidula or boat-shell. 



Subclass II. Euthyneura (Land Snails and many naked Mollusks). — Gastero- 



