ioi8 PRINCIPLES OF STRATIGRAPHY 



while Chiton and its aUies cling to stones, shells, etc., and are 

 rare in deep water, where only their more archaic representatives 

 occur. A few species of Dentalium occur in moderately shallow 

 water, but most of them live below the hundred fathom Hne, some 

 reaching a depth of 2,000 fathoms or more. Chiton seldom ex- 

 tends below 500 fathoms. In both groups a mero-planktonic larva 

 occurs. 



Gastropoda. The gastropods are typical benthonic animals, 

 inhabiting the sea, fresh water, and the land. They almost invari- 

 ably belong to the vagrant benthos, though the degree of locomotion 

 varies greatly among the different species. Among the exceptions 

 to the general vagrant habit are V^ermetus and some other genera, 

 which lead a truly sedentary benthonic life, being attached to rocks 

 or shells. Some genera, like Capulus, adhere continually to shells 

 and the tests of echinoderms and Crustacea ; while the limpets, 

 though adhering powerfully to rocks and shells by the muscular 

 foot, are, nevertheless, in the habit of crawling about in search of 

 food. Swimming and floating gastropods are also known, the 

 latter ( Janthina, Glaucus, etc. ) belonging to the true plankton. 



The number of species living on land and in fresh water is 

 relatively small, though the individuals often occur in great num- 

 bers. The sea is the home of most gastropods, though some marine 

 forms can live in fresh water, while conversely fresh-water forms 

 have been found in water temporarily salt. Thus Limnjea was 

 found by Darwin (10) in Brazil, in a fresh-water lake to which 

 the sea had access at least once a year. 



Among the normally fresh-water gastropods several occur which 

 have become adapted to a marine life. Thus Planorhis glaher was 

 found at a depth of 1,415 fathoms at Cape Teneriffe, and two 

 species of Nerita have been found in the sea. Limnaea and Neri- 

 tina live in the Baltic, where the water contains from 10 to 15 per- 

 mille of salt. 



The variety of form and coloration is exceedingly great among 

 the gastropods, a fact which can easily be correlated with their high 

 degree of cephalization and actively vagrant life. They occupy all 

 parts of the sea, being much less dependent on the facies of the 

 sea bottom than the pelecypods are. The division into carnivorous 

 and herbivorous forms is also much more strongly emphasized than 

 in the pelecypods, which live largely upon the plankton. 



The shore zone is occupied by a number of species, which can 

 withstand periodic exposure. Many of them require this exposure 

 and will invariably crawl to the surface if kept in confinement, 

 even if the water is kept cool and well aerated. Other species, 



