MONOGRAPH OF THE EXISTING CRINOIDS. 651 



Potts found these worms always of a dark, almost black, appearance, and 

 frequenting dark-colored crinoids, and believes that they must be absent from or 

 rare on the lighter-colored hosts. 



Family SERPULID^. 



The tubes of serpulid worms are very commonly found on the cirri and stems 

 of the pentacrinites, and occasionally on the cirri of the comatulids. 



None of the species occurring on the crinoids have been determined, but 

 indefinite references to them are given by Mortensen (cirri of Hathrometra pro- 

 liva), Carpenter, and others. 



MTZOSTOMIDA. 



The Myzostomida, though very highly specialized and aberrant and differing 

 widely in many respects from the other members of the group, belong to the 

 Annelida Polychseta. 



The great majority are parasites of crinoids, but a few species occur upon or 

 within starfishes and ophiurans. 



Many of the myzostomes wander freely over the body of the host, while 

 others are more sedentary; some are more or less permanently fixed in one position, 

 causing enlargements or malformations of the underlying pinnule or arm; a few 

 attach themselves to the ambulacral surface of the pinnule, the segments of which 

 become larger and more hollowed out, thus forming a canal, while the whole 

 pinnule is wound spirally, forming a chamber in which the parasite lives; certain 

 species bore their way into the interior of a pinnule, which becomes swollen and 

 pear shaped, and others cause insignificant thickenings of the brachials with 

 fissures between them; some species cause the formation of small fusiform oval 

 chambers, arranged longitudinally or transversely and arising from the enlarge- 

 ment of some of the brachials or segments of the division series, while still others 

 form cysts upon the ambulacral surface, sessile or of various forms, or pedunculate 

 or club shaped, which are not produced by the transformation of a brachial or 

 pinnule segment. A few species are true internal parasites, living either in the 

 alimentary canal or in the ovaries. 



The body of the myzostome is a more or less circular disk provided on the 

 margin with 10 pairs of digitiform processes. On the ventral side, arranged in 

 two semicircles, are five pairs of nonarticulate parapodia, in the intervals between 

 which, and nearer the margin, are four pairs of suckers. On the end of each of 

 the parapodia is a curved pointed hook supported by a straight rod, which in 

 order to guide the hook is furnished at its extremity with a bent end plate 

 (manubrium) and several smaller hooks. The whole apparatus is capable of 

 extension and retraction by means of a complicated system of muscles radiating 

 outward from a central ventrally placed muscular mass. Close to the anterior 

 end of the ventral surface is the mouth, and close to the posterior end is the aper- 

 ture of the cloaca. The alimentary canal consists of a muscular pharynx, which 

 can be extruded through the mouth, of an esophagus separated by a valve from 



