33(5 MR. A. HANCOCK ON THE ANATOMY 



form we have a Tunicate in whicli development has been arrested, 

 and which, to a certain extent, has an embryonic character. In 

 it the branchial sac is entirely absent, and the circulatory system 

 is much in the same condition as it is in the young of Ascidia 

 before the respiratory organ is developed. 



In Saljpa 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 membrane 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 removed, 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 membra- 

 nous 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, corre- 

 spond to the boundary lines of attachment of the branchial sac of a 

 simple Ascidian ; and if we suppose a vascular network extended 

 from either side of the endostyle to the lateral margins of the 

 " branchial band," and imagine it to be attached in front and behind 

 along the ciliated bands, we shall see how readily a Salpa may be 

 made to assume the most striking feature of an ordinary Ascidian. 



Now the ciliated bands are the homologues of the posterior 

 cord and the lower member of the anterior cord or collar of the 

 branchial sac of the simple Ascidians ; and the "branchial band" 

 is the equivalent of the oral or ventral lamina of the same group. 

 The relation of the ciliated bands, particularly the anterior, to the 



