196 Mr. A. Hancock on the Anatomy of the 



which in Clavelina is frequently of great length. Why then 

 should the entrance to it be considered to correspond to the 

 cardia in Bryozoa ? These as well as the Ascidiae have a well- 

 marked stomach with cardiac and pyloric orifices ; in both, too, 

 there is a distinct oesophagus ; then should not the orifice leading 

 to it be assumed to be the mouth, or analogous to the mouth in 

 both ? In the polype a series of respiratory tentacles, in the 

 Ascidia the branchial sac, surrounds this mouth ; should not these 

 then be considered homologous ? The affirmative of this would 

 appear to be the natural inference in the first instance. But we 

 are referred to the tentacular filaments at the entrance of the 

 respiratory sac as the true representatives of the tentacles of the 

 polype. With the view to ascertain how far this is correct, I 

 examined with much care Ascidia sordida and Molgula arenosa, 

 and found that these tentacular filaments are not anatomically 

 connected with the branchial sac, but are developments from the 

 tunic. The sac terminates a little way below these filaments, and 

 they fringe the inner circumference of the belt of sphincter 

 muscles which guard the respiratory orifice. These tentacular 

 filaments, then, originating in the tunic, cannot possibly be the 

 homologue of the tentacles of the polype, as these undoubtedly 

 belong to the alimentary canal ; but are in fact a new develop- 

 ment in connexion with the sphincter of the tunic, and share its 

 function. The tentacles then of the polype and the branchial 

 sac of the Ascidian would appear to be homologous; — unless 

 indeed the tentacles of the one have died out, and the branchial 

 sac of the other is altogether a new development, which is not 

 by any means likely. 



In confirmation that the former is the fact, we have only to 

 look at the growing bud of the polype, which so closely resembles 

 an Ascidian, particularly when young, that it might at first sight 

 be taken to be one. The tentacles at this time, all lying parallel 

 to each other within the membranous sheath, have quite the ap- 

 pearance of a branchial sac ; and when we take in connexion with 

 it the alimentary canal, the resemblance is almost complete. 

 Indeed, all that is wanting to turn the polype into an Ascidian, 

 so far as the alimentary and respiratary organs are concerned, is 

 the union of the tentacles by a vascular membrane. And we 

 have already seen that such an union has commenced in Frede- 

 ricella, Plumatella and Alcyonella. We have seen that in all 

 these genera the tentacles are united at the base by a delicate 

 membrane ; and in the former this membrane is so extensive as 

 to suggest the idea of a rudimentary form of the branchial sac 

 of the Tunicata. 



Taking this view of the homology of the parts, the longitudinal 

 laminae in the interior of the branchial sac of the Ascidia will 



