W. K. BROOKS ON THE GENUS SALPA. 11 



these two points is considerably less than a fourth of the circumference 

 of the body. In Fig. 3, and in Plate XXXV, they are a little further 

 apart ; still further apart in Fig. 5, while in the adult, Plate I, Fig. 4, they 

 are at opposite ends of the long axis of the body. The history is the same 

 in the aggregated form, as is shown by the series of figures of the aggre- 

 gated Salpa pinnata in Plate VII, Figs. 4 and 5; Plate VIII, Fig. 1, and 

 Plate I, Fig. 3. 



The test of salpa has never received the attention it merits. Most of 

 the figures and descriptions of the species are from preserved specimens, 

 and as the muscles are made opaque and distinct by alcohol, while the 

 test remains transparent, and usually becomes more or less swollen and 

 indefinite by the action of preserving fluids, the published descriptions 

 contain very few references to the fact that in many species the surface 

 of the body is delicately sculptured and is marked by ridges and serrations. 



In this particular most of the figures are untrue to nature, and as I 

 have attempted to show in my figures the structure of the test, many of 

 my illustrations of familiar species are so different from those which 

 usually pass as correct drawings that I feel called upon to explain the dis- 

 crepancy. 



In many species the test is divided by longitudinal ridges or keels 

 into thick and thin portions, and the ridges are sometimes simple and 

 sometimes serrated. Their physiological function is undoubtedly to give 

 strength and stiffness to the test, that it may antagonize the muscles 

 more effectively, and restore the shape of the body after contraction. 



While the different species which possess these ridges exhibit con- 

 siderable variation, there is a general plan which can always be recog- 

 nized. On the middle line of the dorsal surface the test is thin, and when 

 the muscles are contracted, as they usually are in preserved specimens, 

 this thin area, Plate III, Figs. 1, 2, 3 and 5, forms a deep longitudinal 

 furrow, bounded on each side by the prominent, keel-like edge of the 

 thickened portion of the test. Along each side of the body there is 

 usually a dorso-lateral keel, and the thickened portion of the test often 

 forms a prominent "wing" on each side of the body between this keel 

 and the one nearer the middle line, as is shown in Fig. 5 and on the left 

 side of Fig. 1. On the sides of the body the test is thin, and sometimes 

 strengthened by a lateral ridge on each side. On the ventral surface the 

 test is thick, and is bounded at the sides by two ventro-lateral keels, Fig. 

 7, while there is often a median keel on the middle line of the ventral 

 surface. I have found these ridges in the solitary form of Salpa demo- 



