THE SEA SHORES l8l 



shrimps, and sometimes other things, most of which, except 

 the first, are harmless, or at least do no more than steal the 

 food that they collect. 



The very bony crinoids would seem to offer little in the way 

 of food for parasites, yet nearly 150 parasitic or semi-parasitic 

 forms have been described from them. A httle groove runs 

 down the middle of the upper side of the pinnules and the 

 arms of crinoids, and the five grooves from the five rays con- 

 verge to the central mouth. The minute creatures taken from 

 the water are passed down along these grooves in a constant 

 stream which becomes richer and richer as more and more of 

 the victims are delivered to it by the side branches, and at 

 the mouth forms a rich plankton soup. Most of the crinoids' 

 parasites are simply grafters, camping along the sides of this 

 stream and sucking up the soup. About two-thirds of these 

 belong to a curious type of worm, called myzostomes, which, 

 except for two sorts, internal parasites in star-fishes, are entirely 

 confined to them. Crinoids support about two dozen kinds 

 of crustaceans of several different types, a few of which bore 

 into the soft parts but most of which appropriate the food 

 material they collect, either from their ambulacral grooves or 

 from their stomachs. Nearly a dozen kinds of brittle-stars 

 have never been found except upon them, about a dozen kinds 

 of small gastropods bore into them and suck their juices, and 

 they support at least one internal worm and many protozoans. 

 Barnacles, hydroids, sponges, foraminifera, corals, rhabdopleu- 

 rids, tunicates and bivalves, and curious polyzoans grow upon 

 them, using them as a support to maintain themselves above 

 the mud or sand. But crinoids have one distinct advantage 

 over mussels in that fishes never eat them. 



All the sea animals are undoubtedly as complicated in their 

 relationships to others as are the mussels and the crinoids; 

 each feeds upon a more or less extensive fist of organisms, and 

 in its turn serves as a source of food for many others. 



The enemies of the smaller animals are mostly the larger 

 and predaceous ones. The enemies of the plant-like t^pes are 



