14 THE SHELL. 



Melanians, for instance, have delicately digitated mantle margins, 

 these digitations forming no secretion, and sometimes thrown 

 back over the shell. 



The mantle is occasionally largely developed into side lobes 

 "which in Margin ella and Cyprsea are so extended as to be habit- 

 ually thrown up over the external surface of the shell, nearly or 

 completely covering it : in such shells an epidermis is not 

 present. The mantle lobes of Cyprsea are beset with numerous 

 papillae, which seem to partake the function of tentacles as 

 tactile organs (Ixi, 99). In other genera, as in Oliva, the 

 mantle is prolonged into filiform processes before and behind 

 (iii, 46). 



The female Vermetus has the mantle cleft in the middle, 

 according to Lacaze-Duthiers, although there is no corre- 

 sponding cleft in the shell, and in Haliotis a similar mantle 

 cleft impresses a groove in the shell, in which are situated the 

 row of holes characteristic of the genus. The shell of Pleuro- 

 toma also has a sinus corresponding to a cleft mantle. The 

 cause of the sutural sinus of the shell of the American fresh- 

 water genus Schizostoma is as yet unknown; it may be due to a 

 similar cause or it may be sexual. As the genus is restricted to 

 the Coosa River and its neighborhood, I am inclined to think 

 that it is a local disturbance of growth, especially as most of the 

 species could not be distinguished from corresponding forms of 

 Goniobasis except by the lip notch or slit. 



The velum or natatory organ is a temporary lobe of the skin, 

 developed in the larval forms of certain gastropods as well as in 

 pelecypods (xx, 52). 



The skin in the latter class is usually smooth, except that the 

 foot in certain boring species develops siliceous granules. The 

 double siphons are parallel prolongations of the mantle margin 

 in certain bivalve mollusks, the superior siphon being excretory, 

 and the inferior one for inspiration (iii, 50). 



THE SHELL. 



The relation of the shell to the breathing organ is very inti- 

 mate : indeed, it may be regarded as a pneumo-skeleton, being 

 essentially a calcified portion of the mantle, of which the breathing 

 organ is at most a specialized part. In its most reduced form it 

 is only a hollow cone, or plate, protecting the breathing organ 

 and heart, as in Limax, Testacella, Carinaria. Its peculiar 

 features always relate to the condition of the breathing organ ; 

 and in Terebratula and Pelonaia it becomes identified with the 

 gill. In the nudibranchs the vascular mantle performs wholly 

 or in part the respiratory office. In the cephalopods the shell 

 becomes complicated by the addition of a distinct, internal, 



