AND PHTSIOLOaT OF THE TUNICATA. 341 



off the secondary vessels : these are the essential parts of the 

 branchial tissue ; and when we look to its anatomical structure as 

 well as to its mode of growth, we can scarcely doubt that the net- 

 work of the gill is truly vascular. Speaking, therefore, of the 

 branchial sac as a perforated membrane, as is frequently done, 

 gives an erroneous idea of its apparently true nature. 



The longitudinal bars which have been so frequently alluded to, 

 and which lie in a plane a Jittle above the inner surface of the re- 

 spiratory sac, are non-essential parts of the organ, their function 

 apparently being, as previously stated, to protect the surface of the 

 gill, and, by the aid of their cilia, to sweep the alimentary matters 

 towards the oral lamina. They are not always developed : they do 

 not exist in Glavelina ; neither are they apparently present in 

 Ferophora ; and they seem to be absent in several of the compound 

 Ascidians ; in Doliolum they have likewise disappeared. 



It is stated above, that Glavelina is nearly related to Salpa ; but 

 Pyrosoma and Doliolum come much nearer to it in their general 

 structure, as well as in the details of their organization. Unfor- 

 tunately I have never seen either of these two interesting forms ; 

 but, judging from the able descriptions of them by Prof Huxley 

 in the ' Philosophical Transactions,' they both present examples of 

 imperfectly developed gills. In Pyrosoma the secondary vessels are 

 entirely absent, and the primary vessels of the two lateral laminaB 

 of the branchial sac do not reach the endostyle, their development 

 having been arrested before they extended so far across the respi- 

 ratory cavity ; their distal extremities, however, will undoubtedly 

 open into the system of pallial sinuses ; in no other way can the 

 flow of the blood through the gill be explained : the circulation is 

 therefore to this extent embryonic. "The longitudinal bars " of 

 Huxley are the homologues of what have been so designated 

 throughout this communication, and are therefore not to be con- 

 founded with the true vascular portion of the gill. To turn 

 Pyrosoma into a Salpa, little more seems necessary than to arrest 

 entirely the growth of the primary branchial vessels, and to give 

 to each individual a separate test. 



An arrest of development of these vessels is carried to a much 

 greater extent in Doliolum. In this form the secondary vessels 

 have not only disappeared, but the longitudinal bars are also ab- 

 sent, and the primary vessels themselves only very imperfectly 

 developed. The two bands named by Huxley respectively the 

 "epipharyngeal" and " hypopharyngeal " in this curious form, 

 undoubtedly indicate the line of the great ventral channel and 



LINK. PBGC. — ZOOLOOT, VOL. IX. 27 



