38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



vertebrate blood stream, and it is from this source that the myzostomes 

 as well as most of the other parasites 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 geo- 

 graphically the distribution of the classes to which they belong quite 

 regardless of that of their hosts, and apparently, excepting possibly 

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

 parasitic on the most ancient of the living crinoids, there is not the 

 slightest correlation between the systematic position of the parasite 

 and that of the crinoid. 



An undetermined internal worm, a parasitic ostracod, Laphysti- 

 opsis, Anilocra, Cirolana (fig. 60), Cyclotclson (fig. 61), Synalpheus, 

 Periclimcnes, Pont oniop sis, Galathea (fig. 59), Ophiactis, Ophiomasa, 

 Ophiocuthiops, Ophiophthirius, Ophiosphcora, Sabinella and Polynoe 

 are known as parasites on crinoids only in the Indo-Pacific region, 

 though Laphystiopsis, a parasitic ostracod (on fish), Sabinella, Synal- 

 pheus, Periclinienes, Galathea, Anilocra, Cirolana, Ophiactis and 

 Polynoe also occur in the Atlantic. 



Collochercs, Enterognathus, Stylina and Hemispeiropsis are known 

 as parasites on crinoids only from the Atlantic ; but all of these are 

 small and must be especially searched for ; probably all occur in the 

 Indo-Pacific. 



Mortensen's parasitic worm of doubtful affinities is only known 

 from the Antarctic ; but only Notocrinus virilis offers a suitable 

 habitat for it. 



Thus while the myzostomes occur wherever crinoids are found the 

 majority of the other parasites and commensals on crinoids are con- 

 fined to the Indo-Pacific region, though many are very closely related 

 to nonparasitic Atlantic species. The chief reasons for this are 

 probably the absence of a richly developed littoral crinoid fauna in 

 the tropical Atlantic comparable to that in the Indo-Pacific region, 

 and the plating of the ambulacra in most of the tropical Atlantic types, 

 including the littoral species, which renders them unavailable as a 

 source of food to most of the parasitic forms. 



It is interesting to note that, with the exception of the myzostomes 

 and the gasteropods, the great majority of the organisms which are 

 directly or indirectly parasitic upon the crinoids are confined to the 

 littoral zone. The reason for this is probably to be found in the 

 development of side and covering plates along the ambulacral grooves 

 of the pinnules, arms and disc of the crinoids from intermediate and 

 great depths which enables the animals to convert the ambulacral 



