642 ORDER PERITRICHA. 



flattened ; caudal appendage nearly equalling one-half the body in length, 

 flattened and flexible, distally adhesive ; ciliary girdles two in number, the 

 anterior wreath encircling the body at a short distance only from the frontal 

 border, the posterior one subcentral ; endoplast and contractile vesicle 

 posteriorly situated, the former band-like, curved, transversely placed, the 

 latter median, exhibiting at diastole a rosette-like contour ; parenchyma 

 transparent, colourless or pale yellow, enclosing anteriorly numerous 

 spheroidal globules. Length 1-500" to 1-280". 

 HAB. Fresh water, among Lemna. 



This animalcule was originally described by O. F. Miiller under the title of 

 Cercaria turbo, its present generic name being first employed by Nitzsch,* and after- 

 wards by Ehrenberg. By none of these earlier writers have the details of the ciliary 

 system been accurately interpreted, the small size and exceedingly active motions 

 of the species accounting mainly, no doubt, for this discrepancy. In no instance 

 as yet has this type been represented as possessing more than a single ciliary 

 girdle, this being further described as encircling the anterior border. The present 

 author has examined this same animalcule on repeated occasions, and has on each 

 of these determined with facility the presence under normal conditions of two such 

 girdles; the more anterior girdle, composed of slightly larger cilia, encircles the 

 periphery at a short distance only from the anterior extremity, leaving a small, bare, 

 cap-like border, while the more posterior circlet is continuous with the transverse line 

 of constriction that separates the wider and inflated anterior from the narrower and 

 more usually triangular hinder region of the body. At one period only of its 

 existence does the animalcule possess but a single girdle of cilia, this being imme- 

 diately after the process of transverse fission, in which case the old ciliary girdle in 

 each separated segment is retained as the posterior or oral one, and a new anterior 

 wreath subsequently developed between that and the anterior extremity. The 

 close resemblance in both general contour and comportment, that subsists between 

 Urocentum turbo and the Calceolus (Peridiniuni) cypripedium of Professor H. James- 

 Clark has been already referred to in the description given of that species. In 

 recognition of such likeness the author has recently examined examples of Uro- 

 centrum, obtained in the neighbourhood of St. Heliers, Jersey, with the special 

 object of ascertaining whether or not the few distinctive characters possessed 

 by Professor Clark's type were not also represented here. These features, as 

 previously explained, consist of the exceedingly fine ciliation of the entire surface 

 of the body behind the, in Calceolus, single anterior wreath of powerful cirrose cilia, and 

 the composite structure of the caudal appendage, which was found under high 

 magnification to consist of a bundle of agglomerated hair-like flexible setae in place 

 of a single stylate organ, as was at first premised. Specimens subjected to the most 

 rigid examination in both the living condition and as killed with osmic acid and 

 other reagents, betrayed no trace of cilia at any point separate from the two 

 characteristic annular wreaths, while the caudal appendage was equally proof 

 against resolution into a compound fasciculate structure, as reported by James- 

 Clark of Calceolus. This last-named organ was found, however, under increased 

 amplification, to exhibit certain structural peculiarities not previously observed. In 

 place of being rigid and stylate as at first supposed, it was now shown to be 

 flattened and band-like, to possess a considerable amount of flexibility, and to be 

 soft and adhesive at its more attenuate distal extremity. As with Calceolus, 

 the movements of this animalcule consist variously of free rotation in the 

 water to and fro upon its long axis as though mounted on a pendulum, while at 

 other times it darts to and fro in a straight line with great velocity, or it fixes 

 itself by the extremity of its caudal appendage to some solid object and spins 



' Beitrage zur Infusorienkunde,' p. 4, 1817. 



