342 ME. A. HAKCOCK OX THE A^'ATOMT 



oral lamina, bent up in accordance ■with, the peculiar development 

 of the creature. In the Ascidice that have the branchial sac pro- 

 longed behind the mouth, the ventral channel extends likewise 

 behind the mouth, as well as in front of it ; and if we suppose the 

 endostyle to be shortened in these species, and the posterior por- 

 tion of the sac to be consequently drawn backward and upward, 

 the corresponding extremity of the ventral channel would pass 

 up the dorsal side of the pallial or branchial chamber ; and thus 

 this axis of the gill would at once take up the position it occupies 

 in Doliolum : that is, part would be above or in front, and part 

 below or behind the mouth ; part would form a " hypopharyn- 

 geal" band, and part an "epipharyngeal " band. 



Kow the primary vessels or "tubular bars" originate in the 

 sides of these bands, and are, as already stated, very imperfectly 

 developed, extending, as they do, only for a short distance, and 

 then terminating by opening through the lining membrane of 

 the respiratory cavity into the pallial sinuses, just we have sup- 

 posed the similar vessels do in Pyrosoma. The vessels or "bars," 

 however, of the middle portion of the gill, according to Prof. 

 Huxley, do not so terminate, but end in free loops. The branchial 

 sac is, indeed, in such a rudimentary condition that one step more 

 in its degradation and it would entirely disappear, and Doliolum 

 would be scarcely distinguishable from Salpa. 



In Append icularia the gill is wholly absent ; but the oral la- 

 mina is represented by the " ciliated band," which adheres to the 

 ventral surface of the respiratory cavity ; and it is interesting to 

 find that the anterior extremity of this band divides iuto two 

 brancbes, which, passing towards the dorsal region, encircle the 

 cavity a little below the ganglion, just as the anterior ciliated 

 band does in Salpa, as the anterior band or collar does in Ascidia. 



In this interesting form, in which the embryonic characters 

 have become permanent, we see the oral lamina still adhering to 

 the wall of the respiratory cavity, as well as the endostyle and 

 anterior collar or ciliated band. All these parts, then, appear to 

 be equally developments of the lining membrane ; and the gill, 

 which in the higher forms has been described to originate in the 

 oral band, or rather in the great ventral channel, which always 

 accompanies it, must likewise be considered a production of 

 this same membrane, and which, too, we have seen it is connected 

 throughout its development. 



It is not my intention, on the present occasion, to enter at 

 any length on the relation that subsists between the Tunicata 



