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MYZOSTOMIDA
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MYZOSTOMIDA , a remarkable See also:group of small parasitic See also:worms which live on crinoid echinoderms; they were first discovered by Leuckart in 1827. Some See also:species, such as Myzostoma cirriferum, move about on the See also:host; others, such as M. glabrum, remain stationary with the pharynx inserted in the mouth of the crinoid. M. deformator gives rise to a " See also:gall " on the See also:arm of the host, one See also:joint of the pinnule growing See also:round the See also:worm so as to enclose it in a cyst (see fig. E); whilst M. pulvinar lives actually in the alimentary See also:canal of a species of Antedon. A typical myzostomid (see A, B, C) is of a flattened rounded shape, with a thin edge See also:drawn out into delicate radiating cirri. The skin is ciliated. The dorsal See also:surface is smooth; ventrally there are five pairs of parapodia, armed with supporting and hooked setae, by means of which the worm adheres to its host. Beyond the parapodia are four pairs of See also:organs, often called suckers, but probably of sensory nature, and comparable to the lateral sense organs of Capitellids (See also:Wheeler). The mouth and cloacal See also:aperture are generally at opposite ends of the ventral surface. The former leads to a protrusible pharynx (B), from which the See also:oesophagus opens into a wide intestinal chamber with branching lateral diverticula. There appears to be no vascular See also:system. The See also:nervous system consists of a circumoesophageal See also:nerve, with scarcely differentiated See also:brain, joining below a large ganglionic See also:mass no doubt representing many fused ganglia (B). The dorsoventral and the parapodial muscles are much See also:developed, whilst the coelom is reduced mostly to branched spaces in which the genital products ripen. Full-grown myzostomids are hermaphrodite. The male See also:organ (C) consists of a- branched See also:sac opening to the exterior on each See also:side. The paired ovaries See also:discharge their products into a median coelemic chamber with lateral branches (C), often called the uterus, from which the ripe ova are discharged by a median dorsal See also:pore into the terminal region of the rectum (See also:cloaca). Into this same cloacal chamber open ventrally a pair of ciliated tubes communicating by funnels with the coelom (See also:Nansen and Wheeler); these are possibly nephridia, and excretory in See also:function. The Myzostomida are protandric hermaphrodites, being functional See also:males when small, hermaphrodite later, and finally functional See also:females (Wheeler). Small " males " are in some species constantly associated with large hermaphrodites, but according to See also:Beard there are in some cases true See also:dwarf males, comparable to the complementary males described by See also:Darwin in the Cirripedia. The See also:embryology of Myzostoma has been C A, Ventral view of Myzostoma. B, See also:Diagram of Myzostoma, show- See also:ing the nervous and alimen- tary systems. C, Diagram of Myzostoma, showing the genital organs (from v. Graff and Wheeler). a, Cloacal aperture. ar, Arm. c, Cirrus. cl, " Cloaca." See also:coe, Coelom. ct, Swollen pinnule forming a cyst. i, See also:Intestine and its caeca. ls, Larval setae. m, Mouth. studied by Metchnikoff and Beard. Cleavage leads to the formation of an epibolic gastrula and ciliated embryo which hatches as a See also:free-See also:swimming larva remarkably like that of a Polychaete worm (D). The larva is provided with postoral and perianal ciliated bands, and on either side with a bunch of See also:long provisional setae. The mesoderm becomes segmented, and the parapodia subsequently develop from before backwards; but almost all See also:internal traces of segmentation are lost in the adult. The structure and development of the Myzostomida seem to show that they are nearly related to See also:Polychaeta (see See also:CHAETOPODA), though highly modified in relation to their parasitic mode of See also:life. Additional information and CommentsThere are no comments yet for this article.
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