W. K. BROOKS ON THE GENUS SALPA. 39 



as their position is not perfectly radial, but inclined to the axis of the 

 chorda, a section does not lay open the whole length of any one cell from 

 periphery to center. The axis of the chorda is occupied by a mass of 

 granular protoplasm with scattered nuclei. 



In an ordinary ascidian the tail degenerates before the definitive 

 structure of the adult is acquired, but in Doliolum, as Uljanin's figures 

 show, it is retained for a time after the little animal has ceased to be, in 

 other respects, a larva, and has completely acquired all the characteristic 

 structures of the adult Doliolum. 



In this respect Salpa agrees with Doliolum, for the young Salpa 

 hexagon a, in Plate III, Fig. 4, is not a larva, but a perfectly formed 

 young salpa, although it still retains its notochord k and its placenta pi. 



Other species of salpa retain the rudimentary tail still longer, and 

 there is a trace of it in the adult Salpa cordif ormis. Plate IV, Fig. 3, 

 is a young specimen of the solitary form of this species, some time after 

 birth, showing the tail as a distinctly marked outgrowth of the cellulose 

 mantle, with a cavity which is part of the body cavity. There is no trace 

 of the chorda, although comparison with embryos of this species and of 

 Salpa hexagona shows that this process is the same as the tail of Fig. 4 

 of Plate III. It grows smaller compared with the body, as the animal 

 grows up, but it is easily seen even in the adult Salpa cordiformis, Plate 

 IV, Fig. 5. 



In Salpa pinnata the tail reaches considerable size, as is shown in 

 Plate XLI, Fig. 5, but degenerative changes occur in it so early that its 

 internal structure is vague and indefinite. It consists, as is shown at k in 

 Plate XXXV and in Fig. 9 of Plate XIX, of an outer wall of flattened 

 ectoderm cells, a, which is covered by a thin layer of cellulose, and forms 

 the wall of a chamber which has groups of blood corpuscles around its 

 periphery, while its central portion is occupied by a vacuolated mass of 

 irregular cells in process of degeneration, and this species, studied alone, 

 would give little evidence as to its nature, although other species show 

 clearly that it is a tail, and that its central mass is a somewhat degener- 

 ated notochord. It now remains to trace its origin from the body which 

 is marked 30 in Plates XIII and XIV, and 19 in Plate XII. Plates XVI 

 and XVII show it, in a transitional stage, at k. In this embryo all traces 

 of the caudal nerve have disappeared, and the notochord is represented 

 by a sharply bounded mass of cells filled with large vacuoles. I was not 

 able to trace the history of the blastomeres, but as the circular vacuoles 

 make their appearance at about the time when the round blastomeres 



