500 



TUNICATA. 



If, now, the individuals of the Salpa stolon remained in the same 

 position as those of the Pyrosoma stolon, the stolon of the Salpidae 



FIG. 268. Diagram representing a stolon of Salpa as it would appear if no secondary 

 shifting of the individuals were to take place (after BROOKS). P, solitary form 

 (parent-individual) ; /, //, III, first, second and third group of individuals ; b, b", 

 b'", branchial sac (pharynx) ; c", c"', atrium ; d, alimentary canal ; ec, ectoderm ; el. 

 elaeoblast ; en, entoderm (of the connecting strands) ; es, endostyle ; g, gill ; h, 

 heart ; n, nervous system ; o", o'", ovary. 



would be accurately represented by BROOKS' diagram (Fig. 268). Here 

 also the individuals are still arranged in a single row one behind the 

 other, their orientation being that of the 

 parent. The median plane of the stolon and 

 of all the individuals on it coincides with 

 that of the solitary form (P). The only 

 distinction between this stolon and that of 

 Pyrosoma is the larger number of indi- 

 viduals here present. The Pyrosoma stolon 

 has only a limited number of individuals 

 (five), but the stolon of Salpa. consists of 

 several consecutive groups, each comprising 

 50 to 100 individuals all at the same stage of 

 development. The most distal group (///) 

 is the oldest, and contains the most highly 

 developed individuals. (For the sake of 

 clearness, the groups depicted in the diagram 

 are composed of not more than four indi- 

 viduals). These groups, further, are not 

 sharply marked off from one another. Be- 

 tween every two groups there are always a 

 few individuals which, in the degree of their 

 development, form transitions between the 

 two groups. We also find, especially in the 



FIG. 269. Diagram illus- 

 trating the position of 

 the individuals on the 

 stolon in the Salpa-chain. 

 es, endostyle-folds ; h, 

 haemal or ventral side of 

 the buds; I, left side; 

 n, nervous system and 

 neural or dorsal side ; r, 

 right side; st, remains 

 of stolon. 



