These animals are parasitic on Crinoids or Asteroids.[[398]] The single family, Myzostomatidae, contains but two genera, Myzostoma F. S. Lkt. and Stelechopus v. Gr.
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.
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.
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; f1 to f5, 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.
The mouth and anus are usually ventral; but in M. glabrum the anus is dorsal, and in a few species both apertures are carried on to the back by the great development of the ventral surface. The alimentary canal is provided with a protrusible pharynx; the intestine is branched; amongst its branches is the coelom, packed with eggs, and functioning as a uterus (usually called "ovary"). The true ovary is a small mass of cells on each side, a proliferation of the coelomic epithelium covering the intestinal wall. A median continuation of the uterus passes backwards above the rectum, and opens either into it or by an independent pore dorsal to the anus. The "lateral oviducts" of Nansen are nephridia with ciliated funnels opening into the coelom (uterus), and with pores leading into the cloaca on its ventral surface; or, in M. belli Wheeler, opening to the exterior. The two testes are branched, and each sperm-duct opens laterally on a papilla, just outside the third parapodium of each side. Wheeler[[399]] has recently shown that in the young Myzostoma the spermatozoa ripen before the ova, so that it is functionally a male; before the spermatozoa are all discharged the ova mature, and the animal is for a time hermaphrodite; later on, however, when all the spermatozoa are used up, the worm is a female. Beard's "dwarf males" are therefore merely the young of hermaphrodite forms. In cysticolous species each cyst usually contains a large female individual and a small male. In these cases the young one (male) discharges all its spermatozoa before the ova ripen, so that a period of immaturity intervenes and a true hermaphrodite condition is omitted; the animal is at first male, and later female. The Myzostomaria are thus "protandric hermaphrodites."
The affinity of these animals has been much discussed; they superficially resemble the Tardigrada in many anatomical features, and differ greatly from Chaetopoda, but as they possess the characteristic chaetae or parapodia, and pass through a larval stage[[400]] similar to that of the Polychaetes, there is no doubt that they are closely allied to the group, and indeed may be regarded as degenerate Chaetopods. It has been suggested that they form a passage group between them and the Tardigrada; and von Graff forms a group Stelechopoda, to include the Myzostomaria, the Tardigrada, and the Linguatulida.