W. K. BROOKS ON THE GENUS SALPA. 7 



The solitary salpa of each species differs from the aggregated form 

 of the same species in many of the details of its structure, and in many 

 cases the difference is very considerable, but the fundamental plan is the 

 same for all. 



The specific characteristics of each solitary salpa are quite different 

 from those of its corresponding aggregated form, so that there is no way 

 of deciding what specimens of the two forms belong together, except 

 by actually rearing them, or by the discovery in the body of a given 

 chain-salpa of an embryo sufficiently advanced in development to exhibit 

 its adult characteristics ; or else by finding them associated in great 

 numbers free from admixture with other species. The two forms, when 

 mature, are nearly equal in size, and this is sometimes an aid in identi- 

 fying them. 



The divergent modification which has produced the various species 

 has affected the two forms of each species in different ways, but to about 

 the same amount, so that we cannot say that the solitary salpae of 

 different species are less or more differentiated than the aggregated salpae. 



This fact indicates clearly that the separation of the life-cycle into 

 the two generations took place before the species diverged from their 

 common ancestor. 



The general features of the structure of a solitary salpa are well 

 shown in the longitudinal vertical section of an embryo of Salpa pinnata 

 in Plate XXXV, and in the horizontal sections in Plate XIX, and in the 

 embryos of this species in Plate XLI, Figs. 1, 2, 3 and 4, and in the 

 embryo of Salpa hexagona in Plate III, Fig. 4. 



The structure of the aggregated salpa is shown in Plate VIII, where 

 Fig. 1 shows two young aggregated salpa? of Salpa pinnata, and Fig. 2 

 two of Salpa cylindrica. 



As the figure of the solitary Salpa africana in Plate IV, Fig. 2, shows, 

 the body is subcylindrical in shape, and the two orifices, the mouth, r, 

 and the atrial aperture, gr v , which are usually close together in sessile 

 tunicates, are widely separated and are nearly or quite at opposite ends 

 of the body. 



In the solitary salpa? of all species, and in nearly all the aggregated 

 salpae, the mouth is terminal, or at one end of the cylinder. In a few 

 aggregated forms, Salpa cordiformis, for example, Plate IV, Fig. 6, it is 

 not at the extreme end, but on the outer surface. 



Figure 8 of Plate XLV is a vertical section through the oral region of 

 the embryo of Salpa pinnata just before the mouth is formed. The figure 



