48 HOMOLOGIES. 



and communicating with a series of secondary tubes, which pass off from it at each side 

 like the leaflets of a pinnate leaf from the common petiole. The main tube, with its lateral 

 pinnae, thus constitutes an imperfect diaphragm, which divides the great thoracic chamber 

 into an anterior or branchial portion, and a posterior or cloaca) portion. Now the main 

 tube may be obviously compared to the great " branchial sinus " of Clavelina, and the lateral 

 branches to the transverse respiratory bars of that tunicate. The longitudinal bars 

 have totally disappeared, and the posterior extremity of the main tube or " branchial 

 sinus" is continued across the alimentary canal on each side, until it reaches the hremal side 

 of the thoracic chamber, thus causing the mouth, which perforates this tube near its centre, 

 to be related to it and to its lateral branches, exactly as the mouth in the Polyzoa is related 

 to the lophophore and tentacula in these. Another point of correspondence between the gill 

 of DoKoIum and the tentacular crown of the Polyzoa is to be found in the fact, that in BoViolum 

 the " thoracic sinus " is absent, and the remote extremities of the respiratory bars of one side 

 are quite separate from those of the other, and thus present the open condition which charac- 

 terises the tentacular crown in the Polyzoa. The gill of DoUolum thus constitutes the exact 

 link by which the branchial sac of the Ascidiae passes immediately into the tentacular crown 

 of the Polyzoa. 



However interesting the hipiwcrepian Polyzoa may be in directly indicating the relations 

 here dwelt on, the infundibulate genera present no difficulty, for the orbicular lophophore, 

 after all, is but an unimportant modification of the crescentic, and is connected to it by a 

 series of intermediate forms. The arms of the lophophore in Plumatella have already become 

 obsolete in Fredericella, in which, however, the lophophore still retains a bilateral figure, 

 which is rendered still more decided by the presence of the epistome. In the marine genus 

 Lagenella, the epistome has disappeared, but the lophophore still retains a slight 

 bilaterality. Finally, in the fresh-water genus Pahulicella, and most of the marine genera, 

 not only has the epistome disappeared, but all trace of bilaterality has now vanished from the 

 lophophore. 



A comparison of the tunics of the Tunicata with the ccEnoecium of the Polyzoa will 

 render still more obvious the relations here insisted on, and show how easily the structure of 

 the one can be explained by the study of tlie other. M. Milne-Edwards has proved by the 

 anatomy of Clavelina that there exist in the Tunicata three distinct envelopes, which, however, 

 may be variously united with one another in the different 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 Polyzoa ; the middle sac, or " mantle," of 

 the Tunicata to the internal investment or endocyst of the Polyzoa; and the internal 

 or third tunic of the Tunicata, which surrounds the branchial sac, and forms the 

 "thoracic chamber" of Milne-Edwards (and which is divided into two portions, one 

 haemal containing the proper branchial sac, and the other neural, constituting the cloacal 

 chamber), will be equivalent to the tentacular 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 on. The homology of the third or innermost tunic 

 of the Tunicata may perhaps, at fir-st sight, not appear quite so manifest ; it is, however, equally 



* See Huxley, " Observations upon the Anatomy and Physiology of Salpa and Pyrosoma, together 

 with remarks on Doliolum and Appendicularia." 'Phil. Trans.,' 1851. 



