282 Dr. G. J. Allman on the Homology of the Organs 



the tentacula in the Polyzoa are neither the rudimentary tentacula at the en- 

 trance of the branchial sac of the Ascidice, nor the longitudinal bars of this 

 sac, as maintained by those naturalists who have yet recognised in tlie branchial 

 sac oi the Ascidice an organ homologous with the tentacular crown oHhePolycoa; 

 but that their true equivalents must be sought for in the transverse bars, and 

 this is fiurther borne out by the observation of the ascidian embryo in which 

 the longitudinal bars would seem to make their appearance subsequently to the 

 transverse ones ; the respiratory sac tlius passing in the course of its development 

 through a stage more nearly corresponding to the simpler condition which we 

 meet with in the respiratory crown of the Polyzoa. 



In Salpa the languets are reduced to a single one, that, however, which re- 

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

 we have made between these organs and the oral appendage of Plumaiella, the 

 languet nearest the mouth, but on the contrary (if we may judge from its po- 

 sition), the most remote from this part of the animal. It is, however, particularly 

 worthy of attention, that both the existing languet of Salpa and the oral ap- 

 pendage of Fredericella, and the hippocrepian Polyzoa, are quite similarly re- 

 lated to the great nervous ganglion. This ganglion we shaU presently see 

 to be homologous in the Tunicata and Polyzoa, and it is manifestly it, and not 

 the mouth, that determines the place of the persistent languet. 



However interesting the hippocrepian Polyzoa may be in directly indi- 

 cating the relations here dwelt on, the infundibulate genera present no diffi- 

 culty, for tliey exhibit, after all, but an unimportant modification of the former, 

 and are connected to them by a series of intermediate forms. The arms of the 

 lophophore in Plumaiella have only to become obsolete in order to transform this 

 genus into a, Fredericella, in which, however, the lophophore still retains a bilateral 

 figure, which is rendered yet more decided by the presence of the oral valve- 

 like organ. In Laguncula Van Ben., the oral appendage has disappeared, but the 

 lophophore still presents a slight bilaterality. Finally, in the fresh-water genus, 

 Paludicella, and most of the marine genera, not only has the oral appendage 

 disappeared, but all trace of bilaterality has now vanished from the lophophore. 



2. Dermal System. — M. Milne-Edwards has proved by the anatomy of Cla- 

 velina, that there exist in this genus, and probably in all Tunicata,'thTee distinct 



