280 Dr. G. J. Allman on the Homology of the Organs 



We now need only a few unimportant modifications in order to complete the 

 resemblance between the branchial sac of Clavelina and the tentacular crown 

 oi Plumatella ; we have only to imagine the oral extremity of the branchial 

 sinus to be prolonged with its bars for a short distance downwards, so as to 

 surround the mouth, the transverse bars to become free at their extremities, 

 where, opposite to the branchial sinus, they communicate with the " thoracic 

 sinus," the longitudinal bars to be suppressed, and the " languets" to be re- 

 duced to one situated in the immediate vicinity of the mouth ; a series of changes 

 involving no essential modification of structure ; and we shall then have an 

 organ only wanting in a deep crescentic depression of the distal extremity of 

 the branchial sinus, to resemble, even in minute details, the tentacular crown 

 of Plumatella. 



Now nearly all the changes which we have thus hypothetically supposed to 

 take place in Clavelina, in order to convert its branchial sac into the tentacular 

 crown of Plumatella^ do actually occur in other genera of Tunicata, some in 

 one, and some in another. The predominant importance of the transverse over 

 the longitudinal bars of the branchial apparatus in the Tunicata is sufiiciently 

 manifest; in most cases they are larger and more evident than the longitudinal; 

 in Pyrosoma^ as appears from Mr. Huxley's account of this genus,* they are 

 not only the better developed, but tliey alone carry cilia ; the transverse bars, 

 moreover, are constant in all the genera, while the longitudinal actually disap- 

 pear in Saljya and in DoHoliim, unless, indeed, we adopt the ingenious view of 

 Mr. Huxley, who supposes that the lower division (" Epipharyngeal Band" of 

 Huxley) of the gill in Doliolum is homologous with the longitudinal bars in 

 Pyrosoma and other tunicates ; an opinion, however, which is surely opposed 

 by the fact, that in two species of Salpa examined by Savigny, this naturalist 

 has pointed out the existence of a small inferior gill, maintained by Mr. 

 Huxley to be the homologue of the inferior division of the gill m Doliolum,; 

 and yet the superior or constant gill in one of these Salpa' shows at the same 

 time traces of longitudinal bars as in Pyrosoma. In Doliolum, as it would ap- 

 pear from Mr. Huxley's short but interesting account, the superior and inferior 

 divisions of the gill are directly continuous with one another behind ; indeed 

 they are evidently one and the same organ carried across the thoracic chamber 



* Loc. cit. 



