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side of this sinus ; the calyciform membrane in the one by the 

 external membrane of the respiratory sac in the other, and the 

 oral valve-like organ of Plumatella by the " languets" of Cla- 

 velina reduced to a single one. 



2. A similar homological identity was attempted to be 

 shown in the Dermal system, and each of the three sacs de- 

 monstrated by Mr. Milne Edwards to exist in Clavelina was 

 maintained to have its exact equivalent in the Polyzoa : the 

 test of the tunicate would thus correspond to the external in- 

 vestment or ectocyst of the polyzoon; the mantle of the tuni- 

 cate to the internal sac or endocyst of the polyzoon ; and the 

 internal tunic of the tunicate to the tentacular sheath of the 

 polyzoon. It was further maintained, that the external orifice 

 through which the tentacular crown is projected in the Poly- 

 zoa is equivalent to the respiratory and cloacal apertures of 

 the Tunicata united ; and that the point where the intestine 

 opens externally in the Polyzoa corresponds to the point where 

 it perforates the internal tunic in the Tunicata, the small space 

 between the tentacula and their sheath, and which, during the 

 exerted state, becomes obliterated, being equivalent to the 

 cloaca of the Tunicata. 



3. A similar resemblance was shown to exist in the diges- 

 tive system. 



4. It was maintained that the circulatory system of the 

 Tunicata was but a very slight advance on that of the Poly- 

 zoa, while in the tunicate genus, Pelonaia, the absence of a 

 heart reduces this system entirely to the type of the Polyzoa. 

 Throughout the whole of the Tunicata and Polyzoa the great 

 sinus system is identical in both. 



5. The muscles existing in the mantle of the Tunicata are 

 equivalent to corresponding fibres in the endocyst of Polyzoa, 

 and the ultimate fibre is found to be striped in both groups. 



6. It was endeavoured to be shown, that the great nervous 

 ganglion was, both in the Tunicata and Polyzoa, equivalent to 

 the branchial and cephalic ganglia of the higher mollusca fused 



