their Organs of Respiration. 



243 



Whatever may be, however, the shape and general dispo- 

 sition of the sac, the texture of its inner parietes remains 

 essentially the same, and is so very remarkable that several 

 authors, who knew not its purport, have expressed astonish- 

 ment at its beauty. It consists of an infinity of little vessels 

 which cross one another at right angles, and thus weave a 

 network, with quadrangular meshes (Jig. 30. ), that are again 



subdivided by vessels of such 

 tenuity that they elude the un- 

 aided vision, and require the mi- 

 croscope for their discovery. With 

 a little attention it may be per- 

 ceived that the vertical vessels 

 come from the transverse vessels, 

 and that these are connected by 

 their two extremities to two great 

 trunks, also vertical, which occupy 

 one of the sides, or rather the 

 edges, of the sac ; and it is natural 

 to conclude that one of these 

 trunks is the artery, and the other 

 the branchial vein. (Cuvier, Me- 

 moirs, xx. p. 11, 12.); : 28 tusmi 



The meshes of this branchial 

 network are generally, as I have 

 said, nearly square and uniform, 

 yet in the different genera there 

 is exhibited a considerable variety of patterns, some of which 



is • 



Phallusia sulcata of Savigny, opened. 



Pattern of the branchiae in 

 Ascidia pedunculata. 



Pattern of the branchiae; in Ascidia weytiligera. 



it as a point still undetermined, whether the network of the tunicata is 

 vascular or not : he thinks it may be formed of muscular fibres. {Hist. 

 Nat., vol. iv. p. 86.) 



