)~ 



355 



the muscular foot, are, nevertheless, in the habil of crawling 

 about in .search of food. Swimming and floating gastro- 

 pods 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 

 numbers. The sea, however, is the home of most gastro- 

 pods, though some marine forms can live in fresh water as 

 well. 



The variety of form and coloration is exceedingly great 

 among the gastropods, a fact which can easily be cor- 

 related 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 plank- 

 ton. 



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. Others live in shallow water, even if stagnant, 

 and will not stand a long exposure. 



The majority of gastropods are shallow-water forms, 

 though a number of them range to depths of between 1,000 

 and 2,000 fathoms. The deep-sea gastropods are character- 

 ized by faint colors, though often this is counterbalanced by 

 the brilliancy and beauty of the iridescence, and even the 

 non-iridescent abyssal species give out " a sort of sheen which 

 is wanting in their shallow-water allies." (Agassiz, '88, II., 

 p. 63.) The coarse ornamentation by knobs, spines, etc., so 

 common in shallow-water species, does not occur in the deep- 

 sea forms, where the ornamentation is more delicate, though 

 often of exquisite richness and beauty. Gastropods feed- 

 ing on vegetable matter are wanting in the deep sea, 

 where no vegetable matter occurs, except what is brought 



