356 



down as Rediment. The food of deep-sea molluscs is largely 

 confined to soft-tissued animals, since thick shells and other 

 hard armors are generally absent in these depths. Agassiz 

 states that the Pleurotomidse outnumber any other group of 

 molluscs in the abyssal fauna. These gastropods are char- 

 acterized by a notch in the outer lip near the suture, this 

 serving for the discharge of the refuse, thus preventing foul- 

 ing of the water used for respiration. Some of these are 

 provided with hollow barbed teeth and poison fangs, which 

 they use to kill their prey. This apparatus "is even more 

 fully and generally developed in the related group of the 

 Conida?, few of which reach any great depth."* A few 

 gastropods are viviparous (Paludina vivipara, Littorina 

 rudis), producing their young in an advanced state of 

 development. 



In nearly all the marine gastropods a veliger larva occurs, 

 the velum being commonly large, wing-like, and fringed with 

 cilia. This velum maybe retained until the shell is long past 

 the protoconch stage. While in most marinegast ropods the 

 veliger larva leads a mero-planktonic existence, some marine 

 forms ( Fulgur, Sycotypus) and the oviparous land and fresh- 

 water gastropods pass through their veliger stage within 

 the egg capsule, losing the velum and other larval organs 

 before passing from the capsule, which they leave as young 

 gastropods with well-developed shells. 



In the case of the marine forms cited, the velum, though of 

 no use as a locomotor organ to the animal, is vi-vy large, 

 and is lost only just before the embryo leaves the egg-cap- 

 sule. In terrestrial and fresh-water forms, on the other 

 hand, the velum is reduced to a single ring of cilia or to two 

 lateral ciliated streaks (Lang, II., p. 257); while in some 

 terrestrial species it is wanting entirely. It is obvious that 

 the distribution of species I hus deprived of a temporary pela- 

 gic life must be more restricted, other things being equal, 

 than thai of species having a free veliger stage of greater or 

 less dural ion. 



* Agassiz, '88, II.. p. 66. 



