356 



clown as sediment. The food of deep-sea molhises is lariiely 

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

 hard armors are generally absent in these dei)tlis. Aj^-assiz 

 states that the Plenrotomid.e ontnnmbei- 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 })oison fangs, A^ hich 

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

 fully and generally develoi)ed in the related group of the 

 Conidfe, 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 marine gastropods the 

 veliger larva leads a mero-planktonic existence, some marine 

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

 water gastropoils 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 oi-gan to the animal, is very large, 

 and is lost only just l)efore the embryo leaves the eg-g-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 \vanting entirely. It is obvious that 

 the distribution of species thus deprived of a temporary pela- 

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

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

 less duration. 



* Agassiz, '88, H., p. 6(5. 



