Myzostomida

Myzostomida are a remarkable group of small parasitic worms which live on crinoids, a type of echinoderm. They were first discovered by Rudolf Leuckart in 1827.

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A typical myzostomid is of a flattened rounded shape, with a thin edge drawn out into delicate radiating hairs called cirri. The dorsal surface is smooth, with five pairs of parapodia on the bottom surface. These parapodia are 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. These organs are probably of sensory nature, and are comparable to the lateral sense organs of Capitellids. The mouth and cloacal opening are generally at opposite ends of the bottom surface. The former leads to a protrusible pharynx, 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. 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 hermaphrodites. Their internal organs consist of a branched sac opening to the exterior or each side. The paired ovaries discharge their eggs into a median chamber with side branches, often called the uterus, from which the ripe ova (eggs) are discharged by a mediar dorsal pore into the end of the rectum.

Some species, such as Myzostoma cirriferum, move about on the host; others, such as Myzostoma glabrum, remain stationary with the pharynx inserted in the mouth of the crinoid. Myzostoma 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 whilst Myzostoma pulvinar lives in the alimentary canal of a species of Antedon.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.