718 



THE AMERICAN NATURALIST 



[Vol. XL1I 



The food of crinoids consists of very small pelagic 

 organisms and minute Crustacea. At or near the surface 

 the crinoid must depend upon those which swim within 

 reach of its pinnules or which it may intercept by the 

 slow movement of its arms; but in deeper water while 

 this source of supply is just as available as at the surface, 

 the crinoid gets, in addition, all the carcasses of those 

 organisms in the levels above it which die and are pre- 

 cipitated to the bottom. The intensity of this rain of 

 food increases proportionately with the depth, so that 

 the deeper a crinoid lives, the greater is the available food 

 supply; consequently, the better nourished will be the 

 individual and the greater its size. 



We see, therefore, that the size of Antedon bifida ap- 

 pears to be merely a question of food supply. Passing 

 from a single species to a consideration of the group in 

 general, we find that the average size gradually increases 

 from the shore line to about the 100 fathom mark; this 

 is plainly due to the gradual increase in the supply of 

 food, as just explained; from 100 fathoms to about 600 

 fathoms the same size is maintained; but below 100 

 fathoms plant life, and with it the host of small organisms 

 dependent directly or indirectly upon it, upon which (as 

 well as upon certain of the minute plants) crinoids are 

 dependent for food, begins to disappear. This gradual 

 disappearance of vegetable organisms and their de- 

 pendents is offset by the gradual increase in the rain of 

 carcasses from above, so that an equilibrium is maintained 

 down to about 600 fathoms, and hence the size of the 

 crinoids remains about the same from the 100 to the 600 

 fathom mark. Below 600 fathoms, the gradual decom- 

 position of the rain of carcasses progressively lessens its 

 food value, and, therefore, we note a decrease in the size 

 <»t the crinoids, hardly noticeable at first, but soon be- 

 <'<>mniu' more marked, until, below 2,000 fathoms, we find 

 <)U h minute coinatulid Bathymetra and the equally 



By tins hypothesis the general absence of the Pen- 



