W. K. BEOOKS ON THE GENUS SALPA. 15 



The "gill" of salpa is a respiratory organ, and a true gill in the 

 physiological sense, but it is not homologous with the structures which in 

 ordinary tunicates are called gills. This name is usually applied to the 

 clefts or slits in the sides of the pharynx by which this communicates 

 with the lateral atria or peribranchial spaces. 



In salpa, as the life-history shows, there is one enormous gill-slit on 

 each side of the body, and the "gill" is simply the portion of the body 

 cavity which lies on the middle line between the pharynx in front and 

 below, and the atrium above and behind, while its sides are the inner 

 edges of the two gill-slits. It therefore corresponds, as Herdman (p. 56) 

 has pointed out, to the structure which in ordinary ascidians he has 

 called the dorsal lamina. I shall give further on my reasons for believing 

 that the gill-slit on each side of the pharynx of salpa has actually arisen 

 by the coalescence of all the gill-slits of an ancestor which had a pharynx 

 like that of ordinary ascidians. At present, however, it has lost all 

 traces of this history, even in the embryo. Herdman says that in 

 Salpa bicaudata there are traces of stigmata along the sides of the gill, 

 but I have made sections through the gill of the embryo of Salpa scutigera, 

 at the stage shown in Plate IV, Fig. 7, and find no trace of stigmata, and 

 according to Traustedt, bicaudata is a synonym for scutigera. 



The central nervous system of salpa is a compact subspherical gang- 

 lion, placed midway between" the mouth and the atrial aperture, on the 

 dorsal surface, in the position which it occupies in the sessile tunicates, in 

 which these two apertures are close together ; and it is so different from 

 the elongated tubular nervous system of the primitive chordate type, as 

 shown, for example, by the larvas of the ascidians, that we are forced to 

 believe that it has been affected by the same influences as those which 

 have led to its centralization in the sessile tunicata. 



In all respects the general plan of the structure of salpa is funda- 

 mentally identical with that of the ordinary tunicates, and the differences 

 are differences of detail. The atrial aperture, instead of being near the 

 mouth, as it is in ordinary tunicates, is widely removed from it, as it is 

 also in Doliolum and Pyrosoma ; and the atrium, instead of being wrapped 

 around the pharynx as it is in the ordinary tunicates and, to a less degree, 

 in Pyrosoma and Doliolum also, is placed end to end with it, but there is 

 no reason whatever for questioning the strict homology of the atrium of 

 Salpa with that of the other tunicates. 



This homology has been questioned by several recent writers on the 

 development of Salpa and Doliolum, but I shall show that the history of 



