W. K. BROOKS ON THE GENUS SALPA. 125 



external opening is soon lost. Folds of the ectoderm of the stolon, 

 Plate XXXIV, Figs. 9, 10 and 11, g and h, soon divide each tube up into 

 a series of vesicles, one on each side of the body of each salpa. Each 

 perithoracic vesicle, Plate XXVI, Fig. 3, E-E', g and h, now acquires 

 an opening or gill-slit by which it communicates with its own half, 

 27 and 28, of the pharjmx, and they also become produced towards the 

 middle line of the dorsal surface, Fig. 1, H-fT, g and h, where they meet, 

 as shown at g and h in Plate XXXIII, Fig. 3, K-K and N-N', and 

 ultimately unite to form the median atrium, Plate XXXVI, Fig. 6, g"', 

 which afterwards acquires a median dorsal aperture, Fig. 5, g\ 



Writers on the embryology of Salpa and allied animals have involved 

 the history of the atrial system in unnatural obscurity, for its origin in 

 the salpa embryo and in the aggregated salpa is in perfect accordance 

 with the teaching of comparative anatomy, and quite irreconcilable 

 with any view except the one which regards these structures in salpa 

 as strictly homologous with the median and lateral atria of ordinary 

 Ascidians. 



It will be necessary to discuss in a later chapter the various views of 

 the writers on the origin and homology of the atrial structures of Salpa, 

 but at present this would lead us too far from our subject. 



I have shown in another place, p. 35, that, as Leuckart pointed out 

 long ago, the atrial aperture of salpa is much nearer the mouth when it 

 first appears than it is later, and that in this respect the ontogeny of 

 salpa exhibits evidence of an Ascidian-like stage in its ancestry. The 

 compactness of the ganglion of salpa, as contrasted with the elongated 

 central nervous system of primitive chordata, and its position between 

 the two apertures of the body, are also features of resemblance to the 

 Ascidians ; and while there are now no traces, at any stage of its develop- 

 ment, of numerous stigmatic gill-slits, like those of Pyrosoma, I shall 

 show soon that there is indirect evidence that they at one time existed 

 in the ancestors of salpa, which are, in this respect, Pyrosoma-like. 



The muscle bands of Salpa are easily intelligible as modified oral and 

 atrial sphincters, and they are distinctly more irregular in the young 

 than they are in the adult. In the young aggregated Salpa cylindrica, 

 Plate VIII, Fig. 2, the fourth and fifth body muscles are clearly shown 

 to arise as branches from an atrial sphincter, and Fig. 1 shows one of the 

 body muscles arising in the same way in the aggregated Salpa pinnata. 



The peculiar anatomical relations of the pharynx and atrium of 

 Ascidians are generally and justly regarded as modifications which were 



