Page:Encyclopædia Britannica, Ninth Edition, v. 21.djvu/14

Rh R T R T the Later English style with embattled parapet. There are a mechanics' institute and a working men's club. Coal and stone are obtained in the neighbourhood, and the town possesses match works and rope and twine factories. The population of the urban sanitary district (area 3302 acres) in 1871 was 3733, and in 1881 it was 5105. ROTIFERA. The Rotifera (or Rotatoria) form a small, in many respects well-defined, but somewhat isolated class of the animal kingdom. They are here treated of sepa- rately, partly on account of the difficulty of placing them in one of the large phyla, partly on account of their special interest to microscopiste. Now familiarly known as " wheel animalcules " from the wheel-like motion produced by the rings of cilia which generally occur in the head region, the so-called rotatory organs, they were first discovered by Leeuwenhoek (I), 1 to whom we also owe the discovery of Bacteria and ciliate Infusoria. Leeuwenhoek described the Rotifer vulgaris in 1702, and he subsequently described Melicerta ringens and other species. A great variety of forms were described by other observers, but they were not separated as a class from the unicellular organisms (Protozoa) with which they usually occur until the appearance of Ehrenberg's great monograph (2), which contained a mass of detail regarding their structure. The classification there put forward by Ehrenberg is still widely adopted, but numer- ous observers have since added to our knowledge of the anatomy of the group (3). At the present day few groups of the animal kingdom are so well known to the micro- scopist, few groups present more interesting affinities to the morphologist, and few multicellular animals such a low physiological condition. General Anatomy. The Rotifera are multicellular animals of microscopic size which present a ccelom. They are bilaterally symmetrical and present no true metameric segmentation. A head region is generally well marked, and most forms present a definite tail region. This tail region has been termed the " pseudopodium. " It varies very much in the extent to which it is developed. It attains its highest development in forms like Philodina, which affect a leech-like method of progression and use it as a means of attachment. We may pass from this through a series of forms where it becomes less and less highly developed. In such forms as Brachionus it serves as a directive organ in swimming, while in a large number of other forms it is only represented by a pair of terminal styles or flaps. In the sessile forms it becomes a con- tractile pedicle with a suctorial extremity. A pseudo- podium is entirely absent in Asplanchna, Triarthra, Polyarthra, and a few other genera. The pseudopodium, when well developed, is a very muscular organ, and it may contain a pair of glands (fig. 2, A, gl) which secrete an ad- hesive material. The surface of the body is covered by a firm homogeneous structureless cuticle. This cuticle may become hardened by a further development of chitin, but no calcareous deposits ever take place in it. The cuticle remains softest in those forms which live in tubes. Among the free-living forms the degree of hardening varies considerably. In some cases contraction of the body merely throws the cuticle into wrinkles (Notommata, Asplanchna) ; in others definite ring-like joints are produced which telescope into one another during contraction ; while in others again it becomes quite firm and rigid and resembles the carapace of one of the Entomostraca ; it is then termed a "lorica." The lorica may be prolonged at various points into spines, which may attain a considerable length. The surface may be variously modified, being in some cases smooth, in others 1 These numbers refer to the bibliography at p. 8. marked, dotted, ridged, or sculptured in various ways (fig. 1, K). The curved spines of Pkilodina aculeata (fig. 1, o.) and the long rigid spines of Triarthra are further develop- ments in this direction. The so-called seta? of Polyarthra on the other hand are more complex in nature, and are moved by muscles, and thus approach the " limbs " of Pedalion. Fio. 1. A, Flotcularia,campanuTata, an adult male, drawn from a dead specimen (after Hudson): t, testis; oc, eye-spots. B, F/oscularia appendicu/ata, an adult female (after Gegenbaur) : a, the ciliated flexible proboscis. C, Stephano- eerot tichhornii : a, the urceolus. D, JUicrocodon clavut, ventral view (after Grenacher) : m, mouth ; a, bristles ; x, architroch ; *, lateral sense-organs. E, Polyarthra platyptera : of, eye-spot; x" isolated tufts representing a eephalo- trocli ; x, branchiotroch ; a, 6, and c, three pairs of appendages which are moved by the muscles m. F, another figure of Potyarthra, to show the position which the appendages may tuke up. G, Philodina aculeata : oc, eye-spot ; s, calcar. H, At tinurus neptuniwt. oc, eye-spot ;, calcar. I, Asplanchna sit- boldii, male, viewed from the abdominal surface : a, anterior short arms; b, posterior longer arms ; m, mouth ; x 1 , cephalotrochic tufts ; T, branchiotroch. J, Atplanchna tieboldii, female; letters as before. K, Noteus guadricornif, to show the extent to which the lorica may become sculptured. (All, except where otherwise stated, from Pritchard.) Several genera present an external casing or sheath or tube which is termed an " urceolus." In Floscularia and StepJianoceros the urceolus is gelatinous and perfectly hyaline ; in ConhchHus numerous individuals live in such a hyaline urceolus arranged in a radiating manner. The urceolus, which is secreted by the animal itself, may become covered with foreign particles, and in one species, the well-known Melicerta ringens, the animal builds up its urceolus with pellets which it manufactures from foreign