MONOGRAPH OF THE EXISTING CBINOIDS. 617 



II. Sedentary species, which rarely, if ever, leave the spot where they have 

 settled, as M. parasitwum. 



III. Cyst-producing species, which cause the formation of galls or swellings on 

 the disk or arms, as M. cysticolum. 



IV. Entoparasitic species inhabiting the digestive tract, as M. pulvinar. 



V. Entoparasitic species living in the ovaries, as Protomyzostoma polynephris. 



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

 ing, play the part of the fleas, lice, jiggers and bots, intestinal worms, and flukes 

 combined as we know them among the land vertebrates. 



In the vertebrates the blood, with its multitudes of red corpuscles which when 

 destroyed are promptly and continuously renewed, is the logical food of practi- 

 cally 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 rich reservoir of con- 

 centrated food. But the uncountable myriads of minute organisms flowing con- 

 tinuously downward along the ambulacral grooves and into the mouth form a 

 stream of nutrient fluid in many ways analogous to the vertebrate blood stream, 

 and it is from this source that the myzostomes, as well as most of the other para- 

 sites, derive their subsistence. 



The species in each group parasitic on the crinoids in those cases in which 

 our information is sufficient to permit us to speak with a reasonable amount of 

 certainty follow bathymetrically and geographically the distribution of the classes 

 to which they belong quite regardless of that of their hosts, and apparently, except- 

 ing possibly in the case of Stelechopus, the most primitive of the myzostomes 

 parasitic on the most ancient of the recent crinoids, there is not the slightest 

 correlation between the systematic position of the parasite and that of the crinoid. 



The following are known as parasites on criuoids only in the Indo-Pacific 

 region : 



Parasitic internal worm. Anilocra. 



Laphystiopsis. Cirolana, 



Parasitic ostracod. Cyclotelson. 



Sabinella. Ophiactis. 



Synalphem. Ophiomaza, 



PericUinenes. Ophiowthiops. 



Pontoniopsis. Ophiophthirius. 



Galathea. Ophiosphcera. 



Polynoe. 



though Laphystiopsis, a parasitic ostracod (on fish), Sabinella, Synalpheus, 

 Periclimenes, Galathea, Anilocra, Cirolana, Ophiactis, and Polynoe also occur in 

 the Atlantic. 



The following are known as parasites on crinoids only from the Atlantic: 



CoHocheres. Hemispeiropsis. 



Stylina. Enter ognathus. 



Holotrichous infusorian. 



