1^18] Michael: BcJiarior of Sal pa <Jr)iiocratica 243 



fore moves forward by jerks along a column of water which passes 

 through its body." 



Within the body at the caudoventral extremity, lies a compact 

 intestinal tract called the nucleus (n). Closely associated with this 

 nucleus in the solitary salpa is an organ called the "proliferous 

 stolon" (st), which is a complicated tubular structure joined at its 

 proximal end to the body just anterior to the nucleus. Distally this 

 stolon segments, thereby giving rise to the aggregate salpae. Each 

 salpa remains attached to the preceding one in the form of a chain 

 and as the stolon continues to segment, the salpae are crowded and 

 pushed along a spiral path encircling the nucleus into the mantle 

 cavity (pi. 9, fig. 2). In fact all except the proximal end of the 

 stolon itself is coiled in the cavity of the test. 



As the number of salpae in the chain increases each undergoes sev- 

 eral changes in position. As the origin and development of the chain 

 is so clearly described by Brooks (1893, p. 78) with reference to 

 Salpa pinnata and *S^. cylindrica, I can do no better than quote from 

 him and thereafter point out in what way S. democratica differs : 



The first indication of the segmentation of the stolon is a series of ecto- 

 dermal folds which first appear at its sides, but soon extend up and down 

 and completely encircle it, and, pushing inwards, mark out the body-cavities 

 of the salpae, and also cut up the tubular structures inside the stolon into 

 segments. 



The active agent in this process of segmentation is the growth of the 

 ectodermal folds, and the other structures are actually cut by these folds. 

 As a result of this process the nerve tube becomes cut up into a series of 

 ganglia, one for each salpa; the perithoracic tubes become cut up into a series 

 of perithoracic vesicles, two for each salpa; the genital string becomes cut 

 up into a series of eggs, one for each salpa,* inclosed in a follicle; and the 

 thickened endodermal epithelium at the sides of the endodermal tube becomes 

 cut up into a series of vertical pouches or pockets, two for each salpa, the 

 rudiments of the right half of the pharynx and of its left half. 



The structures which I have enumerated form the rudiments of a single 

 salpa. At this stage each salpa is bilaterally symmetrical, and the plane of 

 symmetry is the same as that of the stolon, while its long axis is at right 

 angles to that of the stolon, which becomes converted into a single row of 

 salpae, so placed that the dorsal surfaces of all of them are toward the base 

 [proximal end] of the stolon, their ventral surfaces towards its tip [distal end], 

 their right and left sides on its right and left respectively, their oral [anterior] 

 ends at its top or neural side, and their aboral [posterior] ends at its bottom or 

 genital side. 



The single row of salpae becomes converted into a double row which con- 

 sists of a series of right-handed salpae and a series of left-handed ones, placed 

 with their dorsal surfaces out, their ventral surfaces toward the ventral sur- 

 faces of those in the opposite row, and the left sides of those on the right 

 and the right sides of those on the left towards the base of the stolon. In 



* In S. democratica there are three eggs, but two disintegrate. 



