50 HOMOLOGIES. 



same time situated in the interval between the internal and middle tunic ; it is, consequently, 

 within the great " sinus system "* of the Tunicata, which corresponds exactly w ith the peri- 

 gastric space of the Polyzoa. Iti the Polyzoa the two orifices coalescing, the ganglion can 

 no longer occupy the position it held in the Tunicata ; it is therefore carried backwards, and, 

 still bathed in the fluid of the sinus, now becomes situated on the oesophagus, a difference of 

 position which, it will easily be seen, involves no important cliange of relations, and which is 

 necessarily connected with the difference in the arrangement of the other organs in the 

 respective groups. In the Polyzoa, from their constant motions of retraction and exsertion, 

 the ganglion could not occupy the fixed position which it does in the Tunicata, and therefore 

 comes to be situated on tiie polypide itself, all whose motions it then necessarily follows. 



To render more complete our comparison between the Tunicata and the Polyzoa, one 

 interesting point of difference must be here noticed, namely, that while in the Tunicata the 

 first bend of the intestine is always, as originally insisted on by Huxley, turned towards the 

 haemal side,! or that opposite to the ganglion, its whole course in the Polyzoa is as invariably 

 towards the neural or ganglionic side. 



To the uniformity of plan now attempted to be demonstrated among the various members 

 of the molluscoid series, the curious tunicate genus AppendicuJaria affords the most im- 

 portant exception. The singular little animals constituting this genus, which I agree with 

 Hnsley;j: and Gegenbauer§ in viewing as an independent form rather than as the larval state 

 of an ascidian, have the thoracic chamber formed on the ascidian type, but in consequence of 

 the non-development of a branchial sac, or of any form of gill,|| this chamber is not divided 

 into a branchial and cloacal portion, and has only a single external orifice. The mouth opens 

 into it below just as in the Ascidians, but the intestine, instead of opening into any part of 

 the thoracic chamber, runs directly to the outer walls of the sac, and terminates by perforat- 

 ing these walls on the ventral side. Appendicular ia, then, cannot be viewed as forming a 

 connecting link between the Tunicata and the Polyzoa ; it is altogether anomalous, and the 

 most important points in whicli it differs from the normal Tunicata are tliose also which 

 separate it at the greatest distance from the Polyzoa. 



A different view of the nature of the parts now under consideration has been taken by 

 Mr. Huxley. Advocating the homology of the branchial sac of the Ascidian, not with the 

 tentacular crown, but with tlie pharynx of the Polyzoon, he has brought to bear upon this 



* This name lias been given by Hu.xley to tlie whole of the space included between the internal 

 and middle tunics in the Tunicata. It is that through which the blood uneuclosed iu proper vessels 

 vaguely circulates. Loc. c'lt. 



\ Witli this generalisation, however, some recent descriptions (see Gegeubauer on Dollohiin, in 

 Siebold and Kolliker's ' Zeitschrift,' vol. vii, 1856) are scarcely consistent. 



+ ' Phil. Traus.,' 1851. § Siebold and Kolliker's ' Zeitschrift,' Band vi, 1855. 



II Gegenbaner has shown that the thoracic chamber in Appvndindaria is pierced just above the 

 mouth by two ciliated apertures, from each of which he traced a tube leading into the interior of the 

 bodv, but was unable to follow it to its destination. It is quite certain that these apertures are in no 

 way homologous with the branchial stigmata of the Ascidiaus ; it is not improbable that they form the 

 entrance to an aquiferous system in this anomalous Tunicate. 



