ANNELIDA - MYZOSTOMARIA The Myzostomaria are small aberrant annelids living as ex ternal or internal parasites on echinoderms. The great majority of their hosts are crinoids ; a few are among the ophiuroids and asteroids. Originally attributed by Leuckart to the trematodes, the Myzostomaria have been put by various authors among the turbellarians, the leeches, and even among the Crustacea; but their trochophore larva is undoubtedly annelidan, and they are now regarded as a modified branch of the Polychaeta.
The body is discoid, oval, or more rarely ob long, furnished along its edge with usually ten pairs of more or less well-developed cirri. On the ventral (very rarely on the lateral) surface there are five pairs of parapodia provided with acicula and hooks ; the latter consist of a stout hooked chaeta sup ported by a bristle with a membranous or chitinous expansion at its tip. There are normally four pairs of suckers, homologous with the lateral sense-organs of the Polychaeta, and alternating with the parapodia. An anterior ventral mouth leads into an un armed retractile pharynx provided with a bulbus musculosus. The pharynx is followed by a stomach and an intestine ; the stomach sends off a varying number of caeca. There is a terminal cloaca.
The body cavity is reduced to a space above the mid-gut and a pair of latero-ventral cavities ; the former serves as an ovisac from which comes off a duct opening into the common cloaca; the latter give rise to the testes and their ducts which end in penis-papillae at the root of the third parapodium of either side. The circulatory system consists of a series of intercellular lacunae filled with coelomic fluid. The nervous system, consisting of a ventral nerve-cord and an oesophageal loop, is of the rope ladder kind. Except in Protomyzostomum there is only a single pair of nephridia.
Von Stummer-Traunfels recognizes four fam ilies: The Myzostomidae, with 106 species; the Protomyzosto midae, with 3 species; the Mesomyzostomidae, with 1 species; and the Stelecjiopidae with i species. They are distinguished by the number of intestinal caeca, position of the parapodia, etc.
The Myzostomaria climb by means of their hooks on the disc and arms of their hosts or form a kind of gall upon them : sometimes they are internal parasites and live in the diges tive tract or the gonad of their host. They are normally protandric hermaphrodites. The young animal functions as a male and then, when the sperm has been discharged and the eggs are ripe, it be comes a female ; when the eggs ripen before the total emission of the sperm a true hermaphroditic phase ensues; otherwise there is an immature stage between the male and female conditions. Certain species carry on their backs young forms still functioning as males.
The best account of the group with an extensive Bibliography. The best account of the group with an extensive bibliography is that of von Stummer-Traunfels in the Handbuch der Zoologie, W. Kukenthal and T. Krumbach, Band III., pp. 132-21o, text-figs. (1926) . P. Fauvel gives a good and concise account of the European forms in "Polychetes sedentaires," etc. Faune de France. xvi., pp. 445-450, text-figs. (1927). (C. C. A. M.)
