NO. 7 SEA-LILIES AND FEATHER-STARS CLARK 35 



the tissues of the host; or (3) while Hving externally upon the sur- 

 face of the body and not feeding directly upon the tissues or fluids 

 of the host are more or less permanently fixed in position and cause 

 more or less extensive malformations, sometimes becoming encysted. 

 This class includes a few " worms," a number of myzostomes, a 

 few crustaceans, and the parasitic gasteropods. 



II. Semiparasitic commensals. — Animals which feed upon minute 

 organisms and have to a greater or lesser extent adopted the habit of 

 sucking up the food particles from the streams flowing down the 

 ambulacral grooves of the crinoid to the mouth, or of temporarily 

 entering the digestive tube and feeding upon the contained matter. 



This class includes the polynoid and ophiuran parasites, most of 

 the Crustacea, and most of the myzostomes. 



III. Nonparasitic commensals. — Animals which, while usually, or 

 commonly, found living upon or among the crinoids lead an entirely 

 independent existence and for the most part are found living under 

 similar relations with other organisms. 



Here are included the foraminifera, sponges, corals, hydroids, 

 polyzoa, barnacles, tunicates and Rhabdopleura, as well as certain 

 shrimps. 



IV. Casual associates. — Animals which normally occur hiding 

 among, crawling over, or attached to other usually arborescent organ- 

 isms (fig. 62) from which they may or may not derive nourisliment, 

 or which normally occur attached to any available support, and which 

 occasionally stray among or upon, or attach themselves to, the crin- 

 oids, but remain otherwise entirely independent of them. 



This class includes a vast number of organisms of very diverse 

 types. 



As in the case of the other arborescent marine types, and in general 

 among the animals that live by filtering the smaller plankton from the 

 sea water, the crinoids are chiefly subject to indirect parasitism, that 

 is to say, the creatures depending upon them for their existence 

 appropriate the food particles which the crinoids have collected in the 

 ambulacral grooves, or even which they have swallowed, instead of 

 consuming the tissues or body fluids directly. Of the animals which 

 derive a part or all of their nutriment from the body or from the 

 eft'orts of the crinoids about 10 per cent are directly parasitic, and 

 about 90 per cent are indirectly parasitic in varying degrees. 



Of the animals which are parasitic on the crinoids nearly all may 

 be described as casual parasites, for they belong to genera or families 

 other representatives of which are nonparasitic ; that is to say, they 

 are merely particular species which have found an easy existence in 



