of the Cilio-flagellate Infusoria. 273 



they are the chief and characteristic traits of this species. The 

 specific name is derived from the resemblance in the form of the 

 body to the labellum of one of the Orchidaceze, viz. Cijpripedium. 

 It is very common in the freshwater ditches and slow streams 

 about Cambridge,U.S. ; and in the aquarium congregates in great 

 numbers around decaying matter. It varies from ~o to -^^ 

 of an inch in length ; but occasionally adults were found -j-J-o of 

 an inch long. It is probable, however, that the latter were in a 

 preparatory state, just before self-division. The colour is a uni- 

 form light brown, which resides mostly in the derm. 



Contractile Vesicle. — This organ (cv) is so conspicuous, in the 

 species before us at least, that one is apt to wonder why it has 

 not been discovered before. The only reasonable excuse for this 

 seeming delinquency would appear to be, that the animal is so 

 incessantly active and so rapid in its motions, that a large amount 

 of patience could hardly compensate for the want of a quiet 

 subject. Fortunately, at the present day our lenses, even of 

 moderate power, are constructed with such large angles, broad 

 fields, and excellent definition, that the difficulty of keeping the 

 Infusorian in sight, and of getting a clear decided view of its 

 interior, is almost done away with. By strewing the glass slide 

 with abundance of indigo, little lagoons are formed here and 

 there, in which, when the specimens are plentiful, there is no 

 difficulty in finding and confining any particular individual, 

 without the necessity of a thin glass covering. In this way the 

 motions of the body are reduced to a simple revolution on its 

 longer axis, with an occasional inversion, end for end. The eye 

 soon gets accustomed to the rhythmical appearance of any par- 

 ticular region as it comes round at each revolution, so that, by 

 a systematic study of each and every feature, a knowledge of the 

 whole organism may be obtained as readily as in most Infusoria. 

 The contractile vesicle is invariably situated close to the nar- 

 rower, or posterior, end of the body, but at a considerable dis- 

 tance from the ventral, dorsal, or lateral surfaces. At the mo- 

 ment just before systole it has a perfectly globular form (PI. XII. 

 fig. 1 cv), and a very sharp, strongly refracting, conspicuous 

 contour, and occupies rather more than the middle third of the 

 transverse diameter of the body at this point. The systole and 

 diastole are as regular in their recurrence as in any of the 

 ciliated Infusoria, and as conveniently observed. The systole, 

 in perfectly fresh specimens, occurs with perfect regularity once 

 in forty seconds, as numerous and carefully registered observa- 

 tions prove. As in other Infusoria, between diastole and sys- 

 tole, the vesicle is more or less irregular in outline, but gradu- 

 ally approximating to a spherical form. At the moment of sys- 

 tole it rather quickly changes from a broad spheroidal figure to 



Ann. fy Mag. N. Hist. Ser. 3. Vol. xvi. 19 



