TUNICATA. 



371 



at first sight, how it is able to subsist, or secure a supply of nour- 

 ishment adequate to its support ; neither is the structure of the 

 mouth itself, or the strange position which it occupies, at all calcu- 

 lated to lessen the surprise of the naturalist who enters upon the 

 consideration of this part of their economy. The mouth, in fact, 

 is a simple orifice, quite destitute of lips or other extensible parts, 

 and situated, not at the exterior of the body, but at the very 

 bottom of the respiratory sac (Jig. 174 and Jig. 175, g). It is 

 obvious, then, that, whatever materials are used as aliment, they 

 must be brought into the body with the water required for respi- 

 ration ; but, even when thus introduced into the branchial cavity, 

 the process by which they are conveyed to the mouth and swal- 

 lowed still requires explanation. We have before noticed that 

 the interior of the branchial chamber is covered with multitudes of 

 vibratile and closely set cilia, well described by Mr. Lister ;* 

 which, by their motion, cause currents in the water. When these 

 are in full activity, observes that gentleman in the paper referred 

 to, the effect upon the eye is that of delicately toothed oval wheels 

 revolving continually in a direction ascending on the right, and 

 descending on the left of each oval, as viewed from with- 

 out ; but the cilia themselves are very 

 much closer than the apparent teeth ; 

 and the illusion seems to be caused 

 by a fanning motion given to them in 

 regular and quick succession, which 

 will produce the appearance of waves, 

 and each wave answers here to a 

 tooth. 



Whatever little substances, alive or 

 inanimate, the current of water brings 

 into the branchial sac, if not rejected 

 as unsuitable, lodge somewhere on the 

 respiratory surface, along which each 

 particle travels horizontally with a 

 steady, slow, course to the front of 

 the cavity, where it reaches a down- 

 ward stream of similar materials ; and 

 they proceed together, receiving acces- 

 sions from both sides, and enter at 

 last the 03sophagus placed at the bot- 



* Phil. Trans, for 1834. page 378. 



Fig. 175. 



