ANATOMY AND PHYSIOLOGY. 51 



adjacent portions of the inner tunic or mantle. A branch 

 from one of the principal nerves has been traced to the 

 branchial tubercle in one or two species. There is no varia- 

 tion of any consequence in the nervous element in any of the 

 forms examined. 



SALPA AND ITS RELATIONSHIP WITH OTHER TONICATA. 



The organisation of Salpa is highly instructive ; for in 

 this form we have a Tunicate in which development has 

 been arrested,, and which, to a certain extent, has an em- 

 bryonic character. In it the branchial sac is entirely absent, 

 and the circulatory system is in much the same condition as 

 it is in the young of Ascidia before the respiratory organ is 

 developed. 



In Salpa the outer tunic or test appears to be quite free 

 from the inner tunic or mantle, except at the margins of the 

 anterior and posterior orifices, where they seem to be united. 

 The inner tunic and lining membrane, or that which forms 

 the inner wall of the respiratory cavity, are, on the contrary, 

 adherent throughout, spaces only being left for the passage 

 of the blood-currents ; for it is between this tunic and mem- 

 brane that the " sinus-system " is situated. The respiratory 

 cavity corresponds pretty accurately to the pallial chamber 

 of a simple Ascidian were the branchial sac entirely re 

 moved, leaving only the endostyle with its two lateral folds, 

 the ventral or oral lamina, and the connecting cords. 



On examining the great respiratory chamber or cavity in 

 Salpa spinosa, for instance, an endostyle with the two lateral 

 membranous folds, similar in all essentials to that organ in 

 the other Tunicata, is seen adhering to the dorsal wall of 

 the cavity; and the so-called "branchial band" or "gill' 

 is conspicuous on the opposite side, passing forward from the 

 nucleus in an inclined position, the posterior extremity being 

 attached in the vicinity of the mouth, the anterior to the 

 ventral wall of the cavity. The two folds in connexion with 

 the endostyle and the "branchial band" are connected in 

 front by a narrow band (the "ciliated band" of Huxley) that 

 encircles the anterior extremity of the respiratory cavity : 

 and another similar band, or pair of parallel bands, passes 

 from the posterior end of the dorsal folds and terminates 

 near to the posterior extremity of the " branchial band." 

 Thus we observe certain lines or bands which, together with 

 the endostyle, correspond to the boundary lines of attach- 

 ment of the branchial sac of a simple Ascidian; and if we 



