Freshwater Bryozoa, with descriptions of new Species. 179 
the upper extremity of the stomach, and the consequence is that 
the intestine, fig. 2 2, is doubled upon itself a little above the 
enlargement, 7, and the cesophagus, e, is forced down by the side 
of the stomach, g, and turning upwards again is bent into the 
form of an 8. 
Vascular System.—This appears to be entirely wanting in 
these animals: a species of circulation nevertheless exists. I 
have seen on two or three occasions a pretty regular flow of the 
fluid in the visceral cavity of Plumatella and Fredericella. Under 
ordinary circumstances no fluid can be recognized in this cavity, 
from the apparent deficiency of blood-globules or corpuscles of 
any kind. Such however probably exist, but the thickness and 
opacity of the cell-walls are sufficient to prevent the detection of 
minute bodies of this nature. On the occasions alluded to some 
of the tissues of the animal appear to have been ruptured, and 
small fragmentary particles mingling with the contained fluid 
were perceived moving in certain directions. By the aid of these 
particles, which were numerous and of various forms and sizes, it 
was easy to ascertain that the fluid which bathes the polype cir- 
culates in a regular manner within the cavity in which the viscera 
float. There can be no doubt that this circulation is caused by 
the action of cilia which cover the inner surface of the liming 
membrane or tunic, and also clothe the external wall of the re- 
tracted tentacular sheath. The current flowed regularly and 
steadily ; but when the floating particles approached the surface 
of the tunic or tentacular sheath, their motion became accelerated 
in a manner that sufficiently evinced the presence of vibratile 
cilia. Those on the tunic chiefly determined the direction of the 
current, which went with great regularity up one side, crossed 
over at the top of the cell, and then went down the other side ; 
it crossed again in an opposite direction a little below the stomach, 
and so completed the circuit. It was not difficult to ascertain 
that the cilia of the tunic on one side of the cell vibrate upwards, 
on the other side downwards ; and that all those on the tentacular 
sheath vibrate upwards. On one side therefore the currents of 
the sheath and tunic oppose each other; and consequently an 
eddy was visible near the top of the cell. 
It is quite evident then that fluid circulates within the visceral 
cavity. What is the nature of this circulation? Is it merely 
respiratory, or is it nutritive? It can scarcely be considered an 
aérating current, as there is no visible communication between 
this cavity and the external water ; and indeed if an orifice exists, 
it must be minute and under the control of the animal, or the 
protrusion of the polype could not be effected in the manner to 
be afterwards described. It is more hkely to be for the purpose 
of nutrition,—standing, indeed, in the place of a vascular system. 
12* 
