482 TUNICATA. 



surface of the abdomen, and, after having bathed the surface of the 

 viscera, gains the base of the branchial sac. When the contractions of 

 the heart are directed forwards, the ascending current of blood passes 

 along the anterior wall of the abdominal cavity and enters a capacious 

 sinus, situated in front of the respiratory chamber, which gives origin 

 on each side to a series of large transverse vessels ; and these intercom- 

 municating with each other by means of innumerable branches disposed 

 vertically, a rich vascular network is formed, that, after spreading all 

 over the walls of the branchial cavity, pours its blood into another 

 vertical sinus situated at the opposite side of the thoracic cavity, into 

 which is likewise poured the vitiated blood derived immediately from 

 the system. Lastly, the circulating fluid, again diffusing itself between 

 the viscera, descends along the dorsal region of the abdomen and again 

 reaches the heart. "Were the circulation constant in the above direc- 

 tion, as Milne-Edwards observes, it would somewhat resemble that 

 of other Acephalous Mollusca. The heart might then be compared 

 to an aortic ventricle, and the anterior thoracic sinus to a branchial 

 vein. But, owing to the contrary directions of the currents of blood, 

 due to the changing action of the heart, the vessels that during one 

 minute perform the functions of veins, are in the next converted into 

 arteries. 



(1266.) When we consider the fixed and immoveable condition of an 

 Ascidian, and its absolute deprivation of all prehensile instruments 

 adapted to seize prey, it is by no means evident, at first sight, how it 

 is able to subsist, or secure a supply of nourishment adequate to its 

 support ; neither is the structure of the mouth itself, or the strange 

 position which it occupies, at all calculated to lessen the surprise of 

 the naturalist who enters upon the consideration of this part of its 

 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 (fig. 243, and fig. 244, g). 

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

 must be brought into the body with the water required for respiration ; 

 but even when thus introduced into the branchial cavity, the process 

 by which they are conveyed to the mouth and swallowed 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 

 * Phil. Trans, for 1834, p. 378. 



