Some of them move, more or less actively, on the surface of their hosts, others live in a sessile condition between the joints of the arms or pinnules, causing a greater or less malformation thereof, sometimes leading to the formation of a more or less globular cyst like a plant-gall, due to overgrowths of the joint, as in _M. deformator_ v. Gr. and _M. cysticola_ v. Gr.: while _M. pulvinar_ v. Gr. is endoparasitic in the intestine. Two species occur on our common _Antedon_ (_Comatula_) _rosacea_; one, _M. cirriferum_ Lkt., creeps about the oral surface, especially along the food grooves of the disc and arms; the other, _M. glabrum_ Lkt., lives close to the mouth of the Crinoid, so that its pharynx can be inserted into the oesophagus of the host; this species rarely moves from this position, and carries a young one on its back. [Illustration: FIG. 185.—_Myzostoma glabrum_ Lkt., on the disc of _Antedon rosacea_. The hermaphrodite individual (⚥) lies over the mouth of the _Antedon_, and carries on its back at the anterior end a young one (♂) with only male organs fully developed. _ap_, The anal papilla of _Antedon_. × 4.] The Myzostomaria are circular or oval, more or less markedly convex dorsally, flat ventrally; _Stelechopus_, however, which lives on _Hyocrinus_, is elongated. The margin of the body is provided with ten or more pairs of cirri, short (_M. glabrum_) or long (_M. cirriferum_), and the general appearance of the animal is greatly changed in some species by the great elongation of the hinder cirri, into which the viscera may extend (_M. filicauda_ v. Gr.). On the ventral surface are five pairs of small conical "parapodia," arranged, like the internal organs, in a radiate manner. Each parapodium carries a couple of chaetae; one a hook, the other serving as a "guide" for this hook. The four "suckers" on each side are either glandular or sensory organs; and Wheeler considers them homologous with the lateral organs of Capitellids; they are usually little developed in those species which live inside cysts. {343}[Illustration: FIG. 186.—_Myzostoma cirriferum._ (After Lang and v. Graff.) The organs are supposed to be seen by transparency. On the right side the more dorsal organs are shown, and on the left, those lying more ventrally. _a_, Anus; _c_, ten pairs of marginal cirri; _f^1_ to _f^5_, the five parapodia of the left side, each with two chaetae; _i_, the branches of the intestine on the right side; _m_, mouth; _o_, the opening of the oviduct (_od_) into the rectum; _ov_, the uterus or coelom, filled with eggs, occupying the spaces between the lobes of the intestine; _p_, the pharynx (acrecbolic introvert) lying in the pharyngeal sac; _r_, rectum; _s_, the four "suckers" of the left side; these, like the parapodia, really lie on the ventral surface; _st_, stomach; _t_, the branching testis; ♂, the pore of the sperm-duct.]