Freshwater Bryozoa, with descriptions of new Species. 179 



the upper extremity of the stomach, and the consequence is that 

 the intestine, fig. 2 h, is doubled upon itself a little above the 

 enlargement, i, and the oesophagus, e, is forced down by the side 

 of the stomach, g, and turning upwards again is bent into the 

 form of an S. 



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 lining 

 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 

 aerating 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. 



12* 



