32 SALPA. 



The mesoblast gives rise to the muscles of the branchial sack, to the 

 heart, and to the pericardium. The two latter are situated on the ventral 

 side of the posterior extremity of the branchial cavity. 



Branchial sack and alimentary tract. The first development of the 

 enteric cavity has already been described. The true alimentary tract is 

 formed as a bud from the hinder end of the primitive cavity. The remainder 

 of the primitive cavity gives rise to the branchial sack. The so-called gill 

 has at first the form of a lamella attached dorsally to the walls of the 

 branchial sack ; but its attachment becomes severed except at the two ends, 

 and it then forms a band stretching obliquely across the branchial cavity, 

 which subsequently becomes hollow and filled with blood corpuscles The 

 whole structure is probably homologous with the peculiar fold, usually 

 prolonged into numerous processes, which normally projects from the 

 dorsal wall of the Ascidian branchial sack. 



On the completion of the gill the branchial sack becomes divided into a 

 region dorsal to the gill, and a region ventral to it. Into the former the 

 single atrial invagination opens. No gill slits are formed comparable with 

 those in simple Ascidians, and the only representative of these structures is 

 the simple communication which becomes established between the dorsal 

 division of the branchial sack and the atrial opening. The whole branchial 

 sack of Salpa, including both the dorsal and ventral divisions, corresponds 

 with the branchial sack of simple Ascidians. On its ventral side the 

 endostyle is formed in the normal way. The mouth arises at the point 

 already indicated near the front end of the nervous system 1 . 



1 Brooks takes a very different view of the nature of the parts in Salpa. He says, 

 No. 7, p. 322, "The atrium, of Salpa, when first observed, was composed of two 

 " broad lateral atria within the body cavity, one on each side of the branchial sack, 

 "and a very small mid-atrium.... The lateral atria do not however, as in most Tuni- 

 "cata, remain connected with the mid-atrium, and unite with the wall of the branchial 

 "sack to form the branchial slits, but soon become entirely separated, and the two 

 "walls of each unite so as to form a broad sheet of tissue, which soon splits up to 

 "form the muscular bands of the branchial sack." Again, p. 324, "During the 

 " changes which have been described as taking place in the lateral atria, the mid- 

 "atrium has increased in size The branchial and atrial tunics now unite upon 

 " each side, so that the sinus is converted into a tube which communicates, at its pos- 

 "terior end, with the heart and perivisceral sinus, and at the anterior end with the 

 "neural sinus. This tube is the gill. ...The centres of the two regions upon the sides 

 "of the gill, where these two tissues have become united, are now absorbed, so that a 

 "single long and narrow branchial slit is produced on each side of the gill. The 

 "branchial cavity is thus thrown into communication with the atrium, and the upper 

 "surface of the latter now unites with the outer tunic, and the external atrial opening 

 "is formed by absorption." 



The above description would imply that the atrial cavity is a space lined by meso- 

 blast, a view which would upset the whole morphology of the Ascidians. Salensky's 

 account, which implies only an immense reduction in the size of the atrial cavity as 

 compared with other types, appears to me far more probable. The lateral atria of 



