BRYOZOA, WITH DESCRIPTIONS OF NEW SPECIES. 375 
forms and sizes, it was easy to ascertain that the fluid which 
bathes the polype circulates 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 linmg membrane or tunic, and also clothe 
the external wall of the retracted tentacular sheath. The cur- 
rent 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. 
Tt 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 likely to be for 
the purpose of nutrition,—standing, indeed, in the place of a 
vascular system. The fluid must therefore hold in suspension 
the products of digestion. These may be supposed to exude 
through the walls of the intestinal canal, probably from the 
enlarged portion of it in Paludicella, and perhaps also from the 
upper portion of the stomach ; and passing into this circulation 
will go at once to nourish the various organs of the animal, all 
of which are bathed with this vivifying fluid, except the ten- 
tacles, which we shall afterwards see, in all probability, receive 
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