OF ARTS AND SCIENCES : FEBRUARY 14, 1865. 395 



In order that the various points of the proof, that the Peridiniens are 

 undoubtedly animals, may be comprehended in systematic sequence, it 

 seems most desirable to present them under separate sections, each de- 

 voted to some particular vital function. 



Habitat and Form. There is probably no generic difference be- 

 tween the species in question here and those described and figured by 

 Allman, in the third volume of the Journal of Microscopical Science, 

 1856, and by Claparede in the memoir above referred to; but in their 

 specific relations no doubt they are distinct. This (fig. 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 

 (rf), 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 flagellum {Jl) is attached, in the middle line of the broad depres- 

 sion, 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 (ce), correspond to these, 

 the one which precedes should be called the anterior, and the other the 

 posterior end of the body ; and as such they will hereafter be desig- 

 nated in this article. 



There are two shallow furrows which encircle the body ; one {pf), 

 rather broad, passes obliquely backwards and around it just behind its 

 middle, and the other {af), quite narrow, encircles 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 ciHa, but the anterior end is 

 devoid of them, and appears to be covered by a low cap {pc) in the 

 form of the 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 con- 

 spicuous, even during the rapid motions of the animal. This is the 

 contractile vesicle, which will be described presently. In point of sen- 

 sitiveness this Peridinium exhibits it in almost as great a degree as 

 Pleuronema and many other timid Infusoria. These are the most evi- 

 dent and striking features, such as readily attract the attention when 

 the body is in motion ; and, moreover, they are the chief and charac- 

 teristic traits of this species. The specific name is derived from the 



