616 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



parasitized by animals of an inferior organization; but among the fixed marine 

 animals the parasites for the most part belong to a phylum with a superior organ- 

 ization, and sometimes even to the same phylum (ccelenterates parasitic on ccelen- 

 terates, crustaceans parasitic on crustaceans, ophiurans parasitic on crinoids, etc.). 

 The relations between the fixed marine animals and their parasites are thus more 

 nearly the same as those between parasitic flowering plants and their hosts. On 

 land the various animal groups are definitely parasitic or nonparasitic; but many 

 plant families, such as the Scrophulariacese, Santalacese, etc., and even many single 

 genera, such as Pedicularis, Melampyrum^ Gerardia, etc., include both parasitic and 

 nonparasitic si^ecies, just as do many families and genera, such as Synalpheus, Peri- 

 clime7ves, etc., occurring with the fixed marine animals. 



Of the animals which are parasitic on the crinoids, nearly all may be de- 

 scribed as casual parasites, for they belong to genera or families other repre- 

 sentatives of which are nonparasitic; that is to say, they are merely particular 

 species which have found an easy existence in preying upon the crinoids, though 

 this mode of life has not induced any special modification of their structure. 



The three types which are of especial interest are: (1) The gasteropods {Stili- 

 fer, Stylina, Sahinella^ and MelaneUa), (2) EnterognathuH^ and (3) the myzostomes. 



The family Melanellidse, to which Stilifer, Stylina, Sahinella, and Melanella be- 

 long, includes species showing all gradations between free-living nonparasitic tj'pes 

 and shell-less parasites living entirely within the body of the host. As parasites the 

 Melanellidse occur only upon the echinoderms, where, however, they are found on 

 species of all the classes. Most of the parasitic forms, including all of those 

 occurring on the crinoids, are characterized by extraordinarily delicate shells. 

 Some of the species are permanently fi.xed in one position on the body of the host, 

 but others, including all those found upon the crinoids, appear to move about 

 and to bore into different parts of the host. It is not a little curious that, apart 

 from MelnneUa capensis and Sfylina comatulicola, all the species parasitic on the 

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

 pinnulars, instead of to the soft ventral integument. 



Enterognathus occurs only in crinoids, but the family to which it belongs is 

 well known as a parasite (or commensal) of the tunicates, most of the species 

 living in the branchial chamber of these animals. 



The myzostomes form a group of very highly specialized polychtcte 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 myzo- 

 stome, has been reported by Mortensen in the ovarian cavity of Notocrinus virilis. 

 If this really is a myzostome, which is not unlikely, as similarly endoparasitic 

 species occur in starfishes {Asterias, Stolasterins, and Ceranuister) and astrophy- 

 tons {Gorgonocephalus eucnemis and G. arcticus), we find in the crinoids (in 

 accordance with Wheeler's classification) five groups of myzostome species, as 

 follows : 



I. Wandenng species, which move about freely and actively over the body of 

 the host, as Myzostomum cirriferum. 



