l.'M Mr. W. K. Brooks on the 



differentiated into these organs after the stolon is formed. 

 The folds in the ectoderm of the stolon divide the " Seiten- 

 strjinge " into a series of solid masses at the sides of the 

 stolon between the ectoderm and the endoderm. These bodies 

 are equal in number to the future Salp», and not twice as 

 numerous. As each Salpa is constricted off from the tube it 

 carries with it the greater part of one of tliese masses of cells 

 from one side of the stolon and the lesser portion of the one 

 on the opposite side. These two masses are not bilaterally 

 placed in the body, but are on the middle line, the larger one 

 being dorsal or neural and the smaller one ventral or hasmal. 

 The latter gives rise to the heart and to the eleoblast, while 

 the larger one, on the neural surface, gives rise to most of the 

 mesoderm of the chain Salpa and also to a cloacal vesicle 

 which is median and unpaired. 



The vesicle becomes distended, and at two points, one on ' 

 each side of the middle line, it unites with the wall of the 

 branchial sac, and the cloaca and the branchial chamber thus 

 become connected through the two gill-slits, while a similar 

 union with the ectoderm in the middle dorsal line forms the 

 cloacal aperture. Seeliger's account is perhaps as near the 

 truth as one could hope to get by the study of transverse 

 sections of the twisted stolon of Salpa democratica ; but a 

 very little study of sections in other planes in more favourable 

 species will show that he has completely failed to understand 

 the subject and that his account has no permanent value. 



It is not only irreconcilable with my own observations, but 

 also with our knowledge of Pyrosoma, for both Seeliger 

 ('Pyrosoma,' pp. 622-624) and Salensky ('Pyrosoma,' pp. 31- 

 36) state that in this genus the perithoracic system is bilate- 

 rally symmetrical, that each bud has two perithoracic vesicles, 

 which are not dorsal and ventral, but right and left, that each 

 of them unites with its own side of the pharynx to form the 

 gill-slits before the two vesicles unite with each other to form 

 the median atrium, and that this arises, as it does in the 

 aggregated Salpa, on the dorsal middle line by the meeting 

 and union of diverticula from the two vesicles, and that the 

 external aperture arises still later, as it does in Salpa, as an 

 independent aperture on the middle line. 



The perithoracic vesicles are derived, as I find tluit they 

 are in Salpa, from the right and left perithoracic tubes of the 

 stolon ; but, in the primary ascidiozooids at least, these are 

 continuous with the perithoracic tubes of the primary embryo 

 or cynthozooid, where, according to both Kowalevsky and 

 Salensky (pp. 466, 473-475) , the evidence that they arise as 



