of the Tunicata and the Polyzoa. 279 



rudimentary tentacula at the entrance of the respiratory sac in the Aseidice 

 is also apparent, not only from the difference of structure, but from the fact, 

 that while the tentacula of the Polyzoa are in immediate relation with the 

 digestive tube, those of the Aseidice are evidently mere appendages of the 

 internal tunic. It is true, that in accordance with this view, we can find no 

 homologue in the Polyzoa for the tentacula oi the Aseidice ; we must therefore 

 conclude, that these organs have absolutely died out in the Polyzoa, a circum- 

 stance for which we have been already prepared by their disappearance in Salpa 

 and other Tunicates. 



In connexion with the tentacular crown, there is another part of the orga- 

 nization of the Polyzoa for which we have still to find an equivalent, and which, 

 without comparison with the Tunicata, would remain inexplicable, namely, the 

 curious valve-like organ which overhangs the mouth in Fredericella and the 

 hippocrepian Polyzoa. Now this is plainly homologous with the tongue-Hke 

 bodies, the " languets" of Milne-Edwards, which are attached along the bran- 

 chial sinus in Clavelina and certain other Tunicates, and thence project into the 

 interior of the branchial sac, and which in Salpa are represented by a single 

 one. The languet in Salpa is connected with a peculiar ciliated cavity lying 

 immediately at its base, and which seems also to have its representative in the 

 excavation of the lophophore at the base of the oral appendage in Plumatellu 

 and the allied forms ; and through which the cavity of this appendage appears 

 to communicate with the perigastric space. Further observation will, in all pro- 

 bability, prove that the interior of the languets in the Tunicata communicate 

 in these with the great " sinus system,"* which is equivalent with the perigastric 

 space of the Polyzoa. ISIilne-Edwards believes the languets in Clavelina to 

 exhibit a kind of erection, a phenomenon which would suggest as its expla- 

 nation such a communication as that here supposed, and which, at all events, ren- 

 ders still more striking the resemblance between the languets of the Tunicata 

 and the oral appendage of the Polyzoa, an organ which seems to present an 

 analogous phenomenon. In both groups the bodies in question would seem to 

 be organs of special sense, probably of taste. 



* This name has been giyen by Huxley to the whole of the space included between the inter- 

 nal and middle tunics in the Timicata, and through which the blood, uninclosed in proper vessels, 

 vaguely circulates. See HoiLET, loc. cit. 



