THE CROWN ANIMALCULE. 
41 
Circulation and Respiration. — The performance of these 
functions in the Rotifer a is involved in a good deal of 
obscurity., as is shown by the diversity of opinions that have 
been expressed on them by the most careful and judicious 
observers. I have but little light on the subject derived from 
the Stepliconoceros, and must; therefore, consider it with refer- 
ence to the entire class.* 
In almost all Rotifera there is seen towards the hinder ex- 
tremity of the body, usually in contact with the cloaca, a 
bladder, which is often of conspicuous size, sometimes occupy- 
ing half the volume of the abdominal cavity, or even more, 
as in some of the Brachioni. It is known as the contractile 
bladder. Under the eye of the attentive observer it is seen 
gradually and insensibly to be distended, until it attains its 
full dimensions, when, from its extreme clearness, its well- 
defined form (which is generally globular, or nearly so), and 
from the fact that it pushes aside the surrounding viscera, 
it is very conspicuous. No trace of corpuscles, moveable or 
stationary, can be discerned by the most careful scrutiny in 
its contents; yet no one can look on it without feeling quite 
assured that it is distended by a fluid. Over the walls, in 
some species, may be discerned numerous filaments irregularly 
ramifying and meandering. 
When the bladder has attained its utmost degree of distension 
it suddenly contracts on all sides, and wrinkles up to a barely- 
perceptible point. The action does not occupy so much as a 
second ; and as soon as it is accomplished the process of reple- 
tion goes on as gradually as before, and is followed by contrac- 
tion in turn. The act of filling is called the diastole, the 
emptying the systole. The periods of contraction are generally 
uniform in the same individual, but sometimes they are very 
irregular.! The average interval may be perhaps set down 
at about fifteen seconds. 
eighty-four hours, in which, however, they are not quite so advanced as 
mine in the earliest state in which I noticed them, at seven hours after birth 
(plate iv. /). The comparison of these data shows how extremely varying and 
uncertain are the periods of development in different individuals. They differ 
greatly, even when the conditions appear exactly alike ; as in specimens 
hatched in the same live-box, in the same water, from the same brood, 
and on the same day. 
* I must beg my reader’s indulgence if my descriptions appear somewhat 
obscure. I have laboured to be as simple as possible ; but the subject itself 
is intricate and difficult, and cannot be understood without some personal 
acquaintance with the objects themselves, as viewed with the microscope. 
The future papers of this series, with their figures, will help to make the 
subject much more intelligible. 
t They recur most rapidly, so far as my experience goes, in the Monocer- 
cad(e ; thus in Monocerca rattus the contractions are about thirteen per 
minute, and in Mastigoccrca carinata I have counted twenty-five in the minute. 
