NO. 7 SEA-LILIES AND FEATHER-STARS CLARK 37 



crinoids are always attached to the calyx plates, or to the cirrals, 

 brachials and pinnulars instead of to the soft ventral integument. 



The curious copepod Enterognathus occurs only in crinoids, but 

 the family to which it belongs is well known as a parasite (or com- 

 mensal) of the tunicates, most of the species living in the branchial 

 chamber of these animals. 



The myzostomes form a group of very highly specialized polychaete 

 annelids and are the chief parasites of the crinoids, to which animals 

 they are almost exclusively confined. On the crinoids they are, with 

 one possible exception, always ectoparasitic, though they may form 

 soft or calcified cysts within which they are almost completely isolated 

 from the outer world. An organism, possibly a myzostome, has been 

 reported in the ovarian cavity of Notocrinus virilis. If this really is a 

 myzostome, which is not unlikely as similar endoparasitic species 

 occur in starfishes (Asferias, Stolasterias and Ceramaster) and 

 astrophytons (Gorgonoccphalus eucnemis and G. arcticus), we find in 

 the crinoids the five following groups of myzostome species : 



1. Wandering species which move about freely and actively over 

 the body of the host, as Mysostomum cirriferum. 



2. Sedentary species which rarely, if ever, leave the spot where 

 they have settled, as M. parasiticwm. 



3. Cyst-producing species which cause the formation of galls or 

 swellings on the arms or disc, as M. cysticolum. 



4. Entoparasitic species inhabiting the digestive tract, as M. 

 pulvinar. 



5. Entoparasitic species living in the ovaries, as Protomyzostoma 

 polynephris does in the astrophytons. 



Thus in the crinoids we find a single group of animals which, 

 broadly speaking, play the part of the fleas, lice, jiggers and bots, 

 intestinal worms and flukes combined as we know them among the 

 land vertebrates. 



A comparison between the myzostomes and the species of Thrips, 

 occurring only on flowers, is also interesting. 



In the vertebrates the blood with its multitudes of red corpuscles 

 which when destroyed are promptly and continuously renewed is the 

 logical food of practically all the parasites which do not inhabit the 

 intestinal canal. The dilute blood of the crinoids, without structures 

 corresponding to the red corpuscles, has none of the features which 

 make the blood of the vertebrates such a reservoir of concentrated 

 food. But the uncountable myriads of minute organisms flowing 

 continuously downward along the ambulacral grooves and into the 

 mouth form a stream of nutrient fluid in manv wavs analogous to the 



