46 HOMOLOGIES. 



the cal)'x-like membrane adherent to the outer side of the tentacula in FredericeUa and the 

 Hippocrcpian Polyzoa. This correspondence will be rendered more obvious by imagining 

 the branchial sinus to be depressed towards the neural side of the Ascidian, by a rotation 

 round its oral extremity through an angle of 90° in a vertical plane passing through the 

 mouth and anus ; its position from longitudinal will thus be changed to transverse, while 

 the transverse bars will become longitudinal, and the branchial sinus and its bars will then 

 liave the same direction as the exserted lophophore and tentacula of JPhnnatella ; while it is 

 interesting to observe, that during the retracted state of the Polyzoon, the lophophore 

 assumes the constant direction of the branchial sinus in the Tunicate. 



That the tentacula of the Polyzoa are not homologous with the unciliated rudimental 

 tentacula at the entrance of the respiratory sac in the Ascidia? is apparent also, not only 

 from the diiference of structure, but from the fact, that while the tentacula of the Polyzoa 

 arc in immediate relation with the digestive tube, those of the Ascidige are mere appendages 

 of the internal tunic. The real homology of the Ascidian tentacula is, in fact, to be found in 

 the tentacular mantel-fringe of a lamellibranchiate mollusc. It is true that, in accordance 

 with this view, we can find no homologue in the Polyzoa for the tentacula of the Ascidiae ; 

 we must therefore conclude, that these organs have absolutely disappeared in the Polyzoa, a 

 ' circumstance for which we have been already prepared by their absence in Salpa and other 

 Tunicates. 



In connection with the tentacular crown, there is another part of the organization of 

 the Polyzoa for which we have still to find an ecpiivalent, and which without comparison with 

 the Tunicata would remain inexplicable, namely the episfome of the phylactolsematous Polyzoa. 

 Now for the determination of the homological import of this somewhat enigmatical organ, the 

 key is at once afforded by the Tunicata. The epistome is plainly homologous with the tongue- 

 like organs, the "languets" of Milne-Edwards, which are attached along the branchial 

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

 branchial sac. In Salpa, the languets are reduced to a single one ; that, however, which 

 remains in this genus is not, as we might be led to expect from the comparison we have made 

 between these organs and the epistome of Plumatella, the languet nearest to the mouth, but 

 on the contrary (if we may judge from its position), the one most remote from this part of 

 the animal. It is, however, particularly worthy of attention, that both the existing languet 

 of Salpa, and the epistome of the hippocrcpian Polyzoa, are quite similarly related to the 

 great nervous ganglion. This ganglion is certainly homologous in the Tunicata and Polyzoa, 

 and it is manifestly it, and not the mouth, that determines the place of the persistent languet. 



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

 blance between the branchial sac of Clavelina and the tentacular crown of Plumatella ; we 

 have only to imagine the oral extremity of the branchial sinus to be prolonged, with its bars, 

 for a short distance towards the haemal side, so as to surround the mouth, the transverse 

 bars to become free at their extremities, where opposite to the branchial sinus they communi- 

 cate with the "thoracic sinus," the longitudinal bars to be suppressed, and the languets to 

 be reduced to one, situated in the immediate vicinity of the mouth — a series of changes in- 

 volving no essential modification of structure — and we shall then have an organ only wanting 

 in a deep crescentric depression of the distal extremity of the branchial sinus to resemble, 

 even in minute details, the tentacular crown of Plumatella. 



