272 Prof. II. J. Clark on the Animal Nature 



Cilio-flagcllatc Infusoria, as the Peridiniens and their congeners 

 arc designated by Clapavede in his, conjointly with Lachmann, 

 most recent publications upon the Infusoria*. 



In order that the various points of the proof that the Peri- 

 diniens are undoubted animals may be comprehended in sys- 

 tematic sequence, it seems most desirable to present them under 

 separate sections, each devoted to some particular vital function. 



Habitat and Form. — There is probably no generic difference 

 between the species in question here and those described and 

 figured by Allman in the third volume of the 'Journal of Micro- 

 scopical Science/ 1856, and by Claparede in the memoir above 

 referred to ; but in their specific relations no doubt they are 

 distinct. This (PI. XII. figs. 1, 2, 3) has an oblique pyriform 

 outline, more than one-third longer than its greatest breadth, 

 and hollowed on one side by a broad longitudinal depression {<!), 

 extending from the narrower end (p) to a short distance beyond 

 the broadest part of the body. Not far from the narrower end 

 the so-called fiagellum (//) is attached, in the middle line of the 

 broad depression, and is so long as to project beyond the end 

 near to which it is situated. As the narrower end (p) is always 

 the posterior, and the broader end (a) the anterior, in the act of 

 swimming, and the relations of the other parts of the body, 

 such as the position of the mouth (m) and particularly the trend 

 of the oesophagus («?), correspond to these, the one which pre- 

 cedes should be called the anterior, and the other the posterior 

 end of the body ; and as such they will hereafter be designated 

 in this article. 



There are two shallow furrows which encircle the body; one 

 (/>/), rather broad, passes obliquely backwards and around it 

 just behind its middle, and the other (af), quite narrow, encir- 

 cles the broader end just in front of the termination of the broad 

 longitudinal depression above mentioned. The whole of the 

 body posterior to the narrower transverse furrow is clothed with 

 vibratile cilia ; but the anterior end is devoid of them, and appears 

 to be covered by a low cap (pc) in the form of a segment of a 

 sphere. In the young this cap is so shallow as to be readily 

 overlooked during the motions of the animal. Close to the 

 posterior end there is a large, clear vesicle (cv), which is quite 

 conspicuous, even during the rapid motions of the animal. This 

 is the contractile vesicle, which will be described presently. In 

 point of sensitiveness this Peridinium exhibits it in almost as 

 great a degree as Pleuronema and many other timid Infusoria. 

 These are the most evident and striking features, such as readily 

 attract the attention when the body is in motion ; and moreover 



* Clapavede and Lachmann, " Etude sur les Infusoires et les Rhizo- 

 podes," Mem. de Tins. Genevois, tomes v., vi., vii. (1858-1861). 



