152 HYATT, 



the Plumatella must be rotated anteriorly until it is par- 

 allel, as in the Brachiopoda, with the antero-posterior 

 axis of the body ; and this rotation must be towards the 

 dorsal side, since upon this side lie the anus and the prin- 

 cipal ganglia in both animals. After this operation is 

 effected, there is no longer any room for the growth of 

 the arms anteriorly, and they must extend, if developed 

 at all, from the free posterior side of the disk. 



The nearest approach to the Ascidian that can be at- 

 tained by modifying, in a somewhat similar manner, the 

 organization of a Polyzooid, is very clearly exhibited by 

 Prof. Allman in two imaginary sections taken respectively 

 from a Clavellma and a Plumatella.* He has faithfully 

 preserved the natural peculiarities of the Polyzooid, and 

 the comparison of these figures beautifully illustrates his 

 homologies. Nevertheless, they do not appear to prove 

 so close a resemblance as we have just shown to exist be- 

 tween the Terebratula and Plumatella. The arms, the 

 single orifice, the tentacles, the lophophore, and the sim- 

 ple dorsal flexure of the alimentary canal, are necessarily 

 retained in his diagram of the Plumatella. All of these 

 are incompatible with the structure of the Ascidian, the 

 general characters of the homologues of these parts in the 

 latter being widely different. The flexure of the alimen- 

 tary canal in the Ascidian is not simple but twisted on 

 itself, the first flexure being towards the ventral instead 

 of towards the dorsal side as among Polyzoa ; and the only 

 genus, Appendicularia, in which the canal has a simple 

 flexure, as in Polyzoa, has the intestinal opening on the 

 ventral instead of the dorsal side. The lophophore, 

 fringed with tentacles and prolonged into brachial appen- 

 dages, is also so obscurely represented, that it becomes 

 difficult to trace out the homological parts. The construc- 

 tion of the sieve-like gill-sac of the Ascidian with its two 

 orifices for the admission of pure water, and its emission 

 mixed with the excrements, after passing through the 

 meshes of the respiratory bars, could only be imitated in 

 a diagram of a Polyzoon by radically altering the struc- 

 ture of the type. 



* Allman, Op. cit. 



