POLYZOA ; EOWERBANKIA. 441 



been drawn into the pharynx*, escape during the act of swallow- 

 ing ; but they are usually met by one of the tentacula, which 

 bends inwards, and by means of a sharp blow drives them back 

 into the mouth. They do not immediately perish in the stomach, 

 but may sometimes be seen to continue their movements for a 

 considerable period within its cavity. Of the vortex which is 

 drawn by the cilia towards the mouth, a considerable amount 

 passes out between the bases of the arms; and this usually 

 contains the finer particles, which thus escape. Of those which 

 remain within the cone, those of fitting size are received into the 

 pharynx ; and, when they have passed downwards to its termin- 

 ation, a more perfect selection appears to take place, by which 

 some are admitted into the stomach whilst others are rejected. 

 Besides this regular flow of water into the pharynx, the fluid is 

 occasionally introduced in larger quantity, by the sudden dilata- 

 tion of the tube, which quickly returns to its former diameter^ 

 and expels a large part of what it had thus taken in. This 

 movement is not improbably connected with the function of 

 respiration. That a part of the alimentary canal should be 

 specially modified for this purpose, is by no means surprising ; 

 for such a modification, in a somewhat different form, is not at 

 all uncommon in various divisions of the animal kingdom. 

 Thus, in the preceding class we have seen that the entrance to 

 the stomach lies at the bottom of a large cavity lined by the 

 respiratory membrane, over the walls of which currents of water 

 are constantly passing, which supply the digestive organs with 

 alimentary materials, besides effecting the aeration of the blood 

 ( 1058). 



1076. As amongst the true Polypes, we meet with species 

 of Polyzoa, which have the power of forming stony cells, by a 

 deposit of lime in their soft tissues. The cells of some of these 

 Polypes are covered in, when the animal is withdrawn, by a sort 

 of lid or flap, which is provided with muscles for drawing it 

 down upon the mouth of the cell. In others of the compound 

 Polyzoaries, however, the cells overlap one another obliquely; 

 and the orifice is not at their extremity, but rather on one side, 

 so that the operculum has only to be slightly upraised, to allow 



