W. K. BROOKS ON THE GENUS SALPA. 13 



The oesophagus, beginning a little to the right of the base of the "gill," 

 o, runs towards the upper surface of the body to open into the stomach 

 which communicates with one or two blind diverticula. The intestine 

 arises on the lower side of the stomach, and, describing a curve, passes 

 to the left of the oesophagus, and opens into the median atrium at or a 

 little to the left of the middle line above the base of the "gill." 



In the species of the Pinnata group there is no compact "nucleus," 

 and the anatomy of the digestive organs is quite different from that of 

 the other species, and there is great difference between those of the soli- 

 tary and those of the aggregated form. 



In the solitary Salpa pinnata, Plate I, Pig. 5, the solitary Sal pa cha- 

 missonis, Plate I, Fig. 4, the solitary Salpa affinis, and probably the 

 solitary Salpa dolichosema, the intestine, Plate XXXV, p, runs through 

 the gill, and the anus is at the extreme anterior end of the median 

 atrium. In the aggregated Salpa affinis and Salpa chamissonis, Plate 

 VIII, Fig. 6, the digestive tract is coiled upon itself much as it is in 

 ordinary salpae, except that the coils are not bound together into a com- 

 pact nucleus ; but in Salpa pinnata the intestine of the aggregated form 

 is nearly ventral in position, Plate VIII, Fig. 1, p, and the anus is far 

 forward and close to, but on the left of the middle line. The history of 

 its development in this species shows that when it first makes its appear- 

 ance the digestive tract of the aggregated form is like that of ordinary 

 salpae, and that it has the position which is shown in Salpa cylindrica, in 

 Fig. 2, with the intestine, p, crossing to the left of the oesophagus, q, and 

 running towards the dorsal surface to open into the cloaca above the 

 base of the gill. As the aggregated Salpa pinnata grows, the intestine 

 and anus move downwards along the left side of the body, and at the 

 stage shown in Plate VII, Fig. 5, the oesophagus and intestine lie at the 

 same level. The left-hand salpa in this figure has its right side towards 

 the observer, and the oesophagus, q, is seen to run from the pharynx to 

 the stomach, p', while on the left side of the right-hand salpa the intes- 

 tine, p, and anus, p", are shown in almost exactly the same position. At 

 an older stage, Plate VIII, Fig. 1, the intestine, p, seems in surface view to 

 be on the middle line, but sections, Plate XXXVIII, Figs. 52 and 80, p, show 

 that it is actually to the left of the middle line, although very close to it. 

 In view of its history in Salpa pinnata, I think there can be no doubt that 

 the primitive position of the digestive tract in all species of salpa is like 

 that which is shown in Plate VIII, Fig. 2, and that, in this particular, the 

 pinnata-like species have undergone secondary modification. 



