POLYZOA known also as BRYOZOA (from the Greek began, moss, and won, an animal; beetiuse malty of these organisms incrust other animals or bodies like moss), and Gimp BRACIlIATA (front the circumstance that their tentacles are ciliated), are so called from many individuals being united into a colony or polyzoary. Although Dr. Grant, in his Observations on the Structure and Nature of Flustrce, in 1827, and Milne-Edwards and Autlouin, in their Resume des Recherchessur les Animaux sans Verlebres faites aux Iles Chausseg, iu 1828, indubitably, ,showed that these animals more closely resembled, in the details of their organization, the molluseous than the radiate subkingdom, with which they formerly confounded, some of our most esteemed English writers (including plot. Owens) persist in retaining them the polyps, instead of placing them in their true position amongst the molluscoid Most of the polyzoa are microscopic; but as they occur in colonies, they often collect ively form sufficiently conspicuous masses,•and although there is little diversity in the form or structure of the animals themselves. there is much difference in the form, arrangement, and composition of the cells or chambers in which the individual animals reside. " In general," says Mr. Gosse, " the form of the cell is ovate or oblong.; but the general shape is variously modified, being tubular, club-shaped, horn-shaped, cradle shaped, square, etc." The arrangement is often shrub-like, or the cells may be arranged in close series, either adhering in irregular patches, as the lepealbe, or rising into broad, flexible leaves, as the flustrce, or common sea-mats, or in solid strong walls, or coral-like masses, as the escharte, or calcareous sea-mats. Each animal lives freely in its cell, with whose walls it is connected only by means of muscular bands and threads at certain points, and by the covering of the mouth of the cell. The animal may either expand itself to it considerable extent out of the mouth of the cell, or it may be altogether restricted within the latter; its movements being due partly to pressure upon the outer walls, and partly to the muscular bands, which act chiefly as retractors. On examining one of these organisms in the expanded state, the mouth is seen to be sur rounded by a crown of tentacles, which arc most commonly ten or twelve in number, and are clothed with vibratile cilia, which lash the water toward the mouth, and thus create numberless little whirlpools, by which nutritious matter is conducted into the oral aperture of the polyzoa. These ciliated tentacles constitute one of the essential points of difference between these animals and the hydraform polyps, With which they were formerly associated. The mouth !cads to a funnel-shaped cavity or pharynx,
which is succeeded by an msophagus, and a true digestive stomach (between which a muscular gizzard intervenes in certain genera), after which the intestine turns back upon itself, and terminates in an anus near the mouth. In the separate intestine and anal orifice. we have another characteristic distinguishing these animals from•the polyps. At the base of the tentacular circle, just above the anal orifice, is a nervous ganglion, which in all the polyzoa lies ou the re-entering angle, between the two extremities of the intestinal canal. No heart has as yet been discovered, the matters, which result from digestion, percolating through the intestinal walls, and becoming mixed with the fluid in which the viscera floats. According to prof. Allman, three distinct modes of repro duction occur in the polyzoa, viz., by buds or gemmm, by true ova, and by free loco motive embryos. This subject, however, requires further investigation.
Minute appendages, of a very remarkable character, are fixed to the cells of many of the genera. They are termed avicularia, or "bird-head processes," and ribracula, or whiplike spines. The avicularia were described by Ellis, who first noticed them (in his Essay towards a Natural Hi.stoey of the Corallines, 1758), as resembling "a bird's bead with a crooked beak, opening very wide;" they consist of a fixed and a movable nipper, like a crab's claw, the latter being worked by special muscles. These moving beaks have been often observed to seize minute animals; hut as these organs have no power of passim their prey to the mouth, the polyzoa cannot receive nourishment from this source. Mr. Grosse ingeniously suggests that "the seizure of a passing animal, and time holding of it in the tenacious grasp until it dies, may be a means of attracting the proper prey to the vicinity of the mouth." The vibracula consists of a long, slender movable seta or bristle, which, according to Gosse, serves "to rid the animal of intruding vagrants, and to cleanse away accidental defilement, by sweeping across the orifice of the cell." Both these kinds of organs nie of service in determining genera. Excellent magnified repre sentations of tie atiticu/aria and vibracula may be seen en referring to Figs. 13 and 11, in Mr. Rusk's excellent article, POLYZOA, in he English Cycloptedict, to which, as also to that gentleman's Catalogue of Marine Polyzoa in the British Museum, and to prof. Allman's " Report on the Fresh-water Polyzoa," published in the Reports of the British Association for 1850, the reader is referred for further information regarding this remarkable class of animals.