282 



BRANCHIjE of tunicata. 



which separate the festoons. The whole of the plaits are 

 twenty-eight, fourteen on each side, and they are margined 

 by an equal number of great longitudinal vessels. The vessels 

 which compose the tissue are excessively fine ; the transverse, 

 however, less delicate than the others, and not so closely 

 set, accommodate themselves very well by their curvature to 

 the outline of the festoons. This description, I feel, needs 

 the aid of Savigny's figure, of which I gladly avail myself, 

 and I am certain that in few other creatures will you find 

 a structure more wonderfully fashioned. (Fig. 53.*) 



Although the branchial tissue apparently covers the whole 

 inner surface of the sac with a continuous network, yet it is 

 really divided into two halves by a furrow in which the 

 trunks of the blood-vessels lie ; and this structure becomes 

 obvious in some families where, as in Pyrosoma, the inter- 

 space is considerable ; and is still more remarkably obvious 

 in the Salpae, in which, in fact, the branchial vessels are not 

 disposed on the walls of the sac, but occupy the margin of 

 two narrow linear leaflets of very unequal lengths that lie 

 across the cavity. These are formed by a duplicature of 



the inner tunic, and 



Fig. 53. 



Cynthia dione, Savigny. 



Teredines, where there are 



two 



the superior margin 

 is garnished with a 

 close series of little 

 vessels which run 

 parallel to one an- 

 other in a transverse 

 direction ; a form and 

 disposition which, 

 says Lamarck, has 

 very little analogy 

 with what is regard- 

 ed as the respiratory 

 organ in the Asci- 

 diae ; -f but which, 

 on the contrary, Ca- 

 ms seems to think is 

 just the link that 

 connects these with 

 the bivalves, " ap- 

 pearing to constitute 

 the transition from 

 the Ascidias to the 

 branchial la- 



elongated 



* Mem. sur les Animaux sans Vcrtebres, 2de partie, passim. 

 t Anini. s. Vert. iii. 114. 



