ROTIFERA 



(By Prof. A. G. Bourne, Presidency College, Madras.) 



THE Rut if era or Rotator'ni form a small, in many 

 respects well-defined, but somewhat isolated class of 

 the animal kingdom. Tliey are here treated of separately, 

 partly on account of the difficulty of placing them in one 

 of the large phyla, partly on account of their special 

 interest to microscopists. 



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 Rutifer milgaris 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 

 adhesive 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 

 marked, dotted, ridged, or sculptured in various ways (fig. 

 1, K). The curved spines of Philodina aculeata (fig. 1, G) 

 and the long rigid spines of Triarthra are further develop- 



1 These numbers refer to the bibliography at p. 93. 



ments in this direction. The so-called setoc of Polyarthra on 

 the other hand are more complex in nature, and are moved 

 by muscles, and thus approach the " limbs " of Pedaliun. 



FIG. 1. A, Flosctilaria campannlata, an adult male, drawn from a dead specimen 

 (after Hudson): t, testis; oc, eye-spots. B, Floscularia appendiculata, on 

 adult female (after Gegenbaur): a, the ciliated flexible proboscis. C, Stfphano- 

 ceros eichhoriiii : a, the urceolus. D, Microcodon clavus, ventral view (after 

 Grenacher) : m, mouth; a, bristles; x, arcliitroch ; s, lateral sense-organs. E, 

 Polyarthra platyptera: oc, eye-spot; x' ', isolated tufts representing a cephalo- 

 troch; T, brancbiotrocb ; a, 6, and c, three pairs of appendages which are 

 moved by the muscles m. F, another figure of Pvlyarthru, to show the position 

 which the appendages may take up. G, Philodina aculeata: oc, eye-spots; s, 

 calcar. H, Actinunis nrplunius: oc, eye-spots; s, calcar. I, Asplanchrut .-(<- 

 boldii, male, viewed from the abdominal surface; a, anterior short arms; o, 



rterior longer arms : in, mouth; x' , cephalotrochic tufts; j-, brancliiotroch. 

 Asplanchna siebohlii, female; letters as before. K, Xoteus ijttuilricortu's, 

 to show the extent to which the lorica may become sculptured. (AH, except 

 where otherwise stated, from Pritchard.) 



Several genera present an external casing or sheath or 

 tube which is termed an "urceolus." In Floscularia and 

 Stephanoceros the urceolus is gelatinous and perfectly 

 hyaline ; in Conochifus 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 Melictrta ritiyens, the animal builds up its 

 urceolus with pellets which it manufactures from foreign 



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