396 PKOCEEDINGS OF THE AMERICAN ACADEMY 



resemblance in the form of the body to the labelhim of one of the Or- 

 chidacefe, viz. Cypripedium. It is very common in the fresh-water 

 ditches and slow streams about Cambridge ; and in the aquarium con- 

 gregates in great numbers around decaying matter. It varies from 

 sh^ to 2-iTj- of ^^ iiich in length, but occasionally adults were found 

 ■j-^jj of an inch long. It is probable, however, that the latter were in 

 a preparatory state, just before self-division. The color is a uniform 

 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 dis- 

 covered before. The only reasonable excuse for this seeming delin- 

 quency 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 pres- 

 ent 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, are about 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 find- 

 ing 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 appear- 

 ance of any particular 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 narrower, or 

 posterior, end of the body, but at a considerable distance from the ven- 

 tral, dorsal, or lateral surfaces. At the moment just before systole, it 

 has a perfectly globular form (fig. 1, cv), and a very sharp, strongly 

 refracting, conspicuous contour, and occupies rather more than the mid- 

 dle 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 per- 

 fectly fresh specimens, occurs with perfect regularity once in forty sec- 

 onds, as numerous and carefully registered observations prove. As in 

 other infusoria, between diastole and systole, the vesicle is more or less 

 irregular in outline, but gradually approximating to a spherical form. 



