54 BRITISH TUNICATA. 



some of the Cynthiada? ; and, indeed, the structure appears to 

 resemble very closely thnt of the 'hepatic organ in Corella 

 parallelogramma. The "mass of clear cells," the " elseoblast" 

 of Krohn, according to Huxley, may perhaps prove to be the 

 same as the well-known cell-structure before described as 

 coating the alimentary tube in the Aftcidite ; but this is mere 

 conjecture. 



Thus we see how close the relationship is between Salpa 

 and Ascidia ; with ClaveUna, however, the connexion is still 

 more intimate. This is undoubtedly a near ally ; it is only 

 necessary to look to immature specimens to be satisfied of 

 this. When the young ClaveUna is about one twentieth of an 

 inch long, and when the thoracic portion would scarcely be 

 half that length, the thorax is remarkably like the embryo of 

 Sal pa. In profile both have a sub triangular form, the anterior 

 opening* being placed at the angle in front, and the posterior 

 at an angle situated considerably backward, the young of 

 ClaveUna having the two orifices nenrly as far apart as they 

 are in the embryo of Sal pa. At this early period of growth 

 the endostyle is distinctly developed in both forms, and the 

 ganglion and the oral lamina are clearly indicated, also the 

 ciliated band or anterior collar. So far, everything is alike; 

 the general similarity of the respiratory cavity is obvious 

 enough; and were the nucleus in Sal pa produced a little more 

 than it is backwards, the resemblance of the two would be 

 almost complete. But in the young of ClaveUna there are, in 

 addition to what has already been described, the tentacular fila- 

 ments of the incurrent tube, which are now of considerable size ; 

 and the branchial sac has already commenced its development. 



The latter organ, however, is in an exceedingly rudimentary 

 condition : only a single transverse channel or primary vessel 

 on each side of the great ventral channel has made its appear- 

 ance, and does not yet extend much more than halfway across 

 the thoracic or respiratory cavity, on its way (so to speak) to 

 the dorsal channel in connexion with the endostyle. On each 

 side of these growing primary vessels five or six secondary 

 vessels, at right angles to them, have commenced to sprout ; 

 and the distal extremities of all of them, as well as of the 

 primary vessels themselves, open through the lining membrane 

 of the thoracic cavity into the sinus-system between it and the 

 inner tunic. Thus is defined, on either side, the nascent 

 atrium, which is only an extension of the cloaca that had been 

 previously formed. Shortly another primary vessel makes its 

 appearance, extending from the ventral channel and connected 

 laterally with the extremities of the secondary vessels already 



