1020 PRINCIPLES OF STRATIGRAPHY 



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 suture, which serves for the discharge of the refuse, thus pre- 

 venting fouHng of the water used for respiration. Some of these 

 molluscs 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 Conidse, 

 few of which reach any great depth." ( Agassiz-i, ii :66.) 



A few gastropods are viviparous (Palndina vivipara, Littorina 

 rudis), producing their young in advanced state of development. 



In nearly all the marine gastropods a veliger larva occurs, the 

 velum being generally large, wing-like, and fringed with cilia. This 

 velum may be 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, Sycoty- 

 pus), 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 to the animal as a locomotor organ, is very large, and is 

 lost only just before the embryo leaves the egg capsule. In terres- 

 trial and fresh-water forms, on the other hand, the velum is re- 

 duced to a single ring of cilia or to two lateral ciliated streaks. 

 (Lang-19, ii :^57), while in some terrestrial species it is wanting 

 entirely. It is obvious that the distribution of species thus de- 

 prived of a temporary pelagic life must be more restricted, other 

 things being equal, than that of species having a free veliger stage 

 of greater or less duration. 



Land snails generally require a considerable amount of moisture 

 in the atmosphere in order to be able to live an active life. Hence 

 they are found most abundantly near streams and in damp woods 

 and ravines, where they live on the ground or on the vegetation. 

 Nevertheless they can withstand a considerable amount of desicca- 

 tion by burying themselves in the soil or closely clinging to rocks 

 or trees. Even the deserts have their species, which obtain their 

 required moisture chiefly from the dew, and from the succulent 

 plants on which they feed. Hence the presence of snail shells in 

 loess deposits is not necessarily indicative of deposition by water. 

 Some remarkable cases are recorded by Woodward (31 '.14), of the 

 suspension of vitality in snails and their subsequent revivification. 

 Thus a specimen of a desert snail which had been affixed to a card 

 in the British Museum for a period of four years (1846-1850), 

 again came to life upon being immersed in tepid water. 



Pteropoda. The pteropods are marine planktonic molluscs 



