44 
POPULAR SCIENCE REVIEW. 
once. Sometimes tlie tags appeal’ triangular or broadly pear- 
shaped, sometimes nearly cylindrical, or slightly elliptical.* 
Such are the forms of the organs, so far as they can be 
determined. It remains to investigate the function and the 
modus operand i. 
The first question is, what becomes of the contents of the 
contractile bladder at the systole? It is exceedingly difficult 
to trace the outflow of the fluid, and I have watched often 
and anxiously with the utmost care to detect this, in vain. Is 
it discharged into the cloaca, and so effused from the body ? oi- 
ls it poured into the respiratory tubes, and so passed into the 
system ? Analogy would lead one to conclude the former to 
be the case ; but, as I say, it is very difficult to see it. On 
one occasion, however, I saw this decisively. Carefully watch- 
ing a specimen of Notommata clavvlata , having previously 
mingled carmine with the water, I observed that the region 
of the rectum situate between the bladder and the cloacal 
orifice was distinctly puffed out at the instant of the contrac- 
tion, and now and then I discerned, quite indubitably, that 
the minuter atoms of floating carmine were shot away by a 
discharged jet d’cau. Thus the excretory office of the bladder 
was decisively proved. 
Whence, then, comes the fluid that is so discharged ? Some 
have supposed that it is derived by direct and immediate per- 
colation from without, through the cloaca, f Others believe 
that the sac is filled from the tortuous vessels. Dr. Leydig 
adopts this hypothesis. He believes that the water enters the 
general cavity from without, either by endosmose, [j: or by 
minute orifices hitherto undetected, and then mingles with the 
products of digestion, absorbed through the thin Avails of 
* These have been described as of two kinds. I find, liOAvever, that the 
variation depends on the aspect in which they are viewed. In a specimen of 
Brachionus dorcas , I casually obtained a very satisfactory view of one of the 
tags in the line of its long axis, when it Avas manifestly a flattened body. The 
triangular form is, then, that of the flat — the sub-cylindrical that of the edge. 
Cohn, correcting the error of Leydig, has also given this explanation. 
t Mr. Dalrymple suggests that the bladder draws in Avater, and expels it 
again, by the cloacal orifice, and that it is by bringing the blood, by means of 
the ciliary movements of the tags, into immediate contact (the delicate mem- 
branous Avail of the sac intervening) with the air of the Avater, that aeration 
or respiration is performed. M. Perty adopts this explanation. Dr. Cohn 
believes that he proved the existence of this incoming current, as Avell as of 
the outgoing one, in Brachionus militaris, a species peculiarly favourable for 
observation, on account of the enormous development of the bladder. On 
mingling colouring matter Avith the water, Cohn says he Avitnessed an inward 
current during each dilatation, and an outAvard one on each act of contraction, 
alternately. 
t Enclosmosc is that law by wliicli a thin fluid passes through a membrane 
to mingle with a denser fluid inclosed in it. (It is fully described in the 
paper on “ The LoAvest Forms of Life.”— Eo.) 
