of the Tunicata and the Polyzoa. 283 



envelopes, which, however, may be variously united with one another in the dif- 

 ferent genera.* Now all these have their homologues in the Polyzoa; the external 

 sac or test of the Tunicata corresponds to the external investment, or ectocyst, of 

 the Folyzoa ; the middle sac, or mantle, of the Tunicata,to the internal investment, 

 orendocyst, of the Polyzoa; and the internal tunic of the Tunicata, which sur- 

 rounds the branchial sac, and forms the " thoracic chamber" of Milne-Edwards 

 (and which is divided into two portions, one inferior, containing the proper 

 branchial sac, and the other superior, constituting the cloacal chamber), will 

 be equivalent with the tentacidar sheath of the Polyzoa. The homology of the 

 two outer tunics of the Tunicata with the ectocyst and endocyst of the Polyzoa 

 is obvious, and need not here be further dwelt upon ; but the homology of the 

 third or innermost tunic of the Tunicata with the tentacular sheath of the Polyzoa 

 is very important, and will require to be considered more in detail. If we ex- 

 amine this tunic in Clavelina, we shall find that it is continuous with the mantle 

 at the respiratory and cloacal orifices, and becomes attached to the alimentary 

 canal, just behind the mouth and anus. It thus holds to the sixrrounding parts 

 in the Tunicata exactly the same relation that the tentacular sheath or inverted 

 tunic in the Polyzoa does to the corresponding parts of these during the retracted 

 state of the animal. In the Polyzoa there is, properly speaking, but one external 

 orifice, namely, that through which the tentacular crown is projected and re- 

 tracted ; but this is equivalent to the respiratory and cloacal orifices of the Tu- 

 nicata united, and the point where the rectum opens externally in the Polyzoa 

 is not, therefore, as supposed by Van Beneden and others, the homologue of 

 the cloacal orifice in the Tunicata, with the cloacal chamber itself become 

 extinct, — a view which evidently originated in the too exclusive contemplation 

 of thePolyzoon in its exserted state, — but rather corresponds to the point where 

 the rectum penetrates the internal tunic in the Tunicata, and the cloaca in the 

 latter will then be represented by the superior or dorsal portion of the space 

 between the tentacular crown and sheath in the Polyzoa, this space becoming 

 obliterated in the exserted state of the polypide.f 



* See Htixlet, loc. cit. 



■f To the normal structure both of the Tunicata and the Pohjzoa, Appendiadaria presents a 

 remarkable exception. In this singular little Tunicate, as described by Hdxley, the branchije are 

 reduced to a mere rudiment, and while the thoracic chamber formed by the internal tunic is largely 

 VOL. XXIL 2 P 



