TYPE PBOTOCHOBDATA. 635 



"behind the posterior end of the endostyle. This stolon 

 eventually divides into a large number of parts, each of which, 

 after undergoing certain somewhat complicated shiftings of 

 position on the stolon, develops into the sexual individual. 

 The terminal portions of the stolon with the maturer individ- 

 uals from time to time break off and float about, forming 

 what are termed the Salpa-chains, and the constituent individ- 

 uals of the chain, the sexual individuals, becoming mature 

 either while still united with their fellows or after separation 

 from them, start again the life-cycle which may be represented 

 by the following scheme : 



Ovum = Solitary Salpa Chain Salpa Ovum. 



(Non-sexual) (Sexual) 



In another genus belonging to the order, Doliolum (Fig. 

 291), the process is somewhat more complicated. The mem- 

 bers of this genus are barrel-shaped forms, the wide branchial 

 aperture being situated at one end and the atrial aperture at 

 the other ; the stigmata are fairly numerous and arranged in 

 two lateral rows, and the mantle muscle-bands form complete 

 circles around the body, resembling the hoops of the barrel. 

 From the ovum there develops a peculiarly-constructed tailed 

 embryo, which, with the loss of the tail, becomes converted 

 into the non-sexual form (Fig. 291, A) characterized by the 

 possession of nine circular muscle-bands, an otocyst {ot) situ- 

 ated some distance from the nervous system on the side of 

 the body, a ventral stolon {st), and a dorsal posteriorly- 

 directed process {dp). From the stolon a number of buds 

 are produced, which, at an early stage, separate from the 

 stolon and migrate to the dorsal process to which they attach 

 themselves, the migration being accomplished, it is said, by 

 means of amoeboid cells, probably cells of the test, which 

 attach themselves in pairs to the base of each bud and serve 

 to convey it to the dorsal process. Upon this process the 

 buds arrange themselves in three rows, the individuals of the 

 lateral rows developing into forms quite different from those 

 resulting from the development of the buds of the median row. 

 The lateral buds when freely developed are characterized by 

 the possession of a large branchial aperture, which occupies 



