MYZOSTOMIDA  , a remarkable

group of small parasitic worms which live on crinoid echinoderms; they were first discovered by Leuckart in 1827 . Some species, such as Myzostoma cirriferum, move about on the host; others, such as M. glabrum, remain stationary with the pharynx inserted in the mouth of the crinoid . M. deformator gives rise to a " gall " on the arm of the host, one joint of the pinnule growing round the worm so as to enclose it in a cyst (see fig . E); whilst M. pulvinar lives actually in the alimentary canal of a species of Antedon . A typical myzostomid (see A, B, C) is of a flattened rounded shape, with a thin edge drawn out into delicate radiating cirri . The skin is ciliated . The dorsal 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 organs, often called suckers, but probably of sensory nature, and comparable to the lateral sense organs of Capitellids (Wheeler) . The mouth and cloacal aperture are generally at opposite ends of the ventral surface . The former leads to a protrusible pharynx (B), from which the oesophagus opens into a wide intestinal chamber with branching lateral diverticula . There appears to be no vascular system . The nervous system consists of a circumoesophageal nerve, with scarcely differentiated brain, joining below a large ganglionic mass no doubt representing many fused ganglia (B) .

The dorsoventral and the parapodial muscles are much

developed, whilst the coelom is reduced mostly to branched spaces in which the genital products ripen . Full-grown myzostomids are hermaphrodite . The male organ (C) consists of a- branched sac opening to the exterior on each side . The paired ovaries 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 pore into the terminal region of the rectum (cloaca) . Into this same cloacal chamber open ventrally a pair of ciliated tubes communicating by funnels with the coelom (Nansen and Wheeler); these are possibly nephridia, and excretory in function . The Myzostomida are protandric hermaphrodites, being functional males when small, hermaphrodite later, and finally functional females (Wheeler) . Small " males " are in some species constantly associated with large hermaphrodites, but according to Beard there are in some cases true dwarf males, comparable to the complementary males described by Darwin in the Cirripedia . The embryology of Myzostoma has been C A, Ventral view of Myzostoma . B, Diagram of Myzostoma, show- 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."

coe, Coelom . ct, Swollen pinnule forming a cyst . i, 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 free-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 long provisional setae . The mesoderm becomes segmented, and the parapodia subsequently develop from before backwards; but almost all 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 Polychaeta (see CHAETOPODA), though highly modified in relation to their parasitic mode of life .