METAZOA MOLLUSCA. 287 



make a tube with an opening at each end. The shell 

 formed by the mantle grows in the same way, and is a 

 tube open at either end. No. 534 (D. entalis Linn.), 

 shows the animal within the tube. From the larger or 

 anterior end the foot projects. The mouth with the 

 odontophore is at the base of the foot. 



Plate 535 represents the fleshy animal as taken from 

 the shell; fig. i, dorsal view, fig. 2, ventral, and fig 3, lat- 

 eral view. The organ colored dark green in the model 

 (No. 536) is the liver; the kidney is at the forward end 

 of the liver, while the generative organs are on one 'side 

 and are colored yellow. 



HETEROPODA. 



In the Heteropoda the foot is modified in different ways, 

 sometimes being in the form of a vertical fin extending 

 from the ventral side. The variability of the gills is 

 shown by the fact that in this group they are present or 

 absent in species of the same genus and even in speci- 

 mens of the same species. 1 



Atlanta peroni Les. (No. 537), has a glassy shell which 

 is a vertical spiral in the young animal, but which later be- 

 comes flattened in one plane. The last whorl increases 

 rapidly in size, and the aperture is flaring. The fleshy 

 body takes the coiled spiral form of the shell and can be 

 wholly drawn into it and protected by the operculum. 

 Around the outside of the shell there is a thin sharp keel 

 which looks at first glance like a delicate membrane. 



Carinaria cristata has a smooth coiled shell when 

 young (PI. 538, fig. i, left side; fig. 2, right side; en- 

 larged). When older the shell takes the form of a coni- 

 cal cap, seen in fig. 3, with flutings on the outer surface. 

 No. 539 is the shell of the adult of another species, C. 



1 Tryon, Structural and Systematic.Conchology, II, 1883, p. 348. 



