MOLLUSCA. 



255 



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 (Streptoneura) or united in one individual (land snails). 

 Development is mostly indirect. 



FIG. 117. 



FIG. 117. Pecten irradians, a Scallop. From Binney's Gould. 



Questions on the figure. Is this an external or internal view of the 

 shell? Where is the umbo? What is peculiar about the hinge in this case? 

 What is the significance of the lines nearly concentric with the margin? 

 Of the radial lines? 



Subclass I. Strcptoncura. 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 Llttorina, 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 

 Crcpidula or boat-shell. 



