HOMOLOGIES. 



45 



and this is a very important distinction, which from not having been recognised, rendered 

 previous attempts at comparison between the respiratory sac of the Ascidian and the 

 tentacular crown of the Polyzoon indefensible. 



Without a knowledge of the Hippocrepian Polyzoa, it would, perhaps, have been im- 

 possible to arrive at anything like a satisfactory conclusion on this point ; the peculiarities of 

 these Polyzoa, however, afford a key to the clearing up of this difficult subject, and we shall 

 best perceive the relations in question by comparing an Ascidian Tunicate with a Hippo- 

 crepian Polyzoon, a Clavelina, for example, with a Plumatella, a comparison which the 

 accompanying diagrams (figs. 6 — 9) will render easy.* Now it does not need much 

 assistance from the imagination to see in the great branchial sinus of Clavelina, a repre- 

 sentative of the lophophore of Plumatella, while the transverse bars which pass off at either 

 side from this sinus, and are richly ciliated, will correspond to the ciliated tentacula of the 

 Polyzoon; the delicate membrane which constitutes the proper walls of the respirator}' sac, 

 to the interior of which the respiratory bars of the Ascidian are adherent, and which is 

 pierced in the intervals of these bars by the " branchial stigmata," will have its homologue in 



F.o;. ». 



Fis. y. 



t^i 



Fig. 8. Plan of a Hypocrepian Polyzoa — 

 louaitudinal section. 



Fig. 9. Plan of a Hypocrepian Polyzoa- 

 transverse section. 



a. Ectocyst. h. Endocyst. c. Tantacula sbeath. ddd. Perigastric space. f -{■ e. Orifice of cell. 

 g. Tentacula. /. Lophophore. /. Calyx. m. Epistorae. n. Jlouth. o. CEsophagus. p. Stomach. 

 q. Intestine. ;•. Anus. ^s. Cavity of tautacular sheath, u. Ganglion, w. Retractor muscle. 



* In figs. 6 — 11, the same letters are used throughout to indicate homologous parts. 



