618 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



but all of these are small and must be especially searched for ; probably all occur in 

 the Indo-Pacific. 



From the Antarctic only we know Mortensen's parasitic worm of doubtful 

 affinity ; 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 confined to the Indo-Pacific 

 region, though many are very closely related to nonpar asitic 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. 



Many of the species of myzostomes have a much more extended geographical, 

 bathymetrical, and thermal range than any known crinoid genus, or even family. 

 Thus Mysostomum gigas occurs in the Arctic on Heliometra, and in the tropical 

 littoral in Brazil on Tropiometra; M. cysticolum occurs in the tropical littoral in 

 Brazil on Comactinia, in the Gulf of -Alaska in 159 fathoms on Florometra., in 

 southern Japan on the species of Calometridse, and on the shores of the Antarctic 

 continent in 500 fathoms on Anthometra; M. vastum occurs in the Caribbean Sea 

 on Comactinia, and in southern Japan on Comanthus; M. excisum occurs in Florida 

 on Coccometra, and in Mauritius on "Antedon impinnata"; M. inflator occurs at 

 Barbados on Neocomatella, and at the Kei Islands on Adelometra; M. carinatum 

 occurs in the Caribbean Sea, at Mauritius, and in the Moluccas: M. elegans occurs 

 in Florida and in the Philippines; and M. murrayi occurs at the Kermadec and 

 Kei Islands, and at St. Vincent, West Indies. Myzostomum japonicum ranges 

 from 52 to 943 fathoms, and M. cornutum from the shore line in the Philippines 

 down to 775 fathoms off southern Japan. 



Generally speaking, the range of SynalpJieus comatularum, Galathea elegans, 

 Ophiom-asa cacaotica, and the parasitic Polynoe coincides with that of the large 

 East Indian comasterids among which they live. 



Laphystiopsis is interesting in being represented by two very closely related 

 species one from the China Sea, parasitic on comatulids, the other from Norway, 

 possibly parasitic on fish. 



Of the gasteropods Stilifer occurs in the Pacific on Bathymetra and in the 

 Atlantic on Bythocrinus, ranging from 237 to 1,772 fathoms; Melanella occurs in 

 South Africa on Annametra, in the West Indies on Orinometra, and off Nova 

 Scotia on RMzocrinus, ranging from the shore line to at least 218 fathoms; and 

 Sabinella occurs at Ceylon and Singapore, and off the Queen Charlotte Islands, 

 ranging from the tropical littoral to 1,588 fathoms. 



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 plates 

 and covering plates along the ambulacra of the pinnules, arms, and disk in most 



