SALPA DEVELOPMENT OF THE BUDS ON THE STOLON. 501 



younger stages, that the distal individuals of a group are somewhat 

 further developed than the 

 proximal individuals of the 

 same group. 



The individuals are in reality 

 arranged on the stolon in two 

 alternate rows (Fig. 269) in 

 such a way that the correspond- 

 ing individuals of the two rows 

 turn their ventral (haemal) 

 sides (h) to each other, while 

 their dorsal (neural) sides (n) 

 are directed outward. This 

 arrangement can be clearly 

 made out in the transverse 

 section (Fig. 275) through an 

 older part of a stolon. We 

 thus have an arrangement of 

 the individuals like that de- 

 picted in the diagram (Fig. 

 269), The median plane of the 

 individuals does not coincide 

 with that of the stolon, but 

 lies at right angles to it. This 

 biserial arrangement of the 

 buds results from a lateral 

 shifting of the segments of the 

 stolon which move alternately 

 to the right and left side. 

 Kach bud at the same time 

 rotates round its longitudinal 

 axis, passing through an angle 

 of 90 ; consequently, the dorsal 

 side of the bud which origin- 

 ally (Fig. 268) was directed 



Ki<;. 270. Horizontal longitudinal section through an advanced stolon of tialpa 

 (after BROOKS). The individuals are cut through obliquely, the longitudinal section 

 passing through them in such a way that, in those placed most distally (1, 2, etc.), 

 the most anterior region of the body is seen and, in the most proximal individuals 

 (20, 21, etc.), the posterior region of the body. 1), distal ; P, proximal ; R, right ; 

 L, left, a, anal aperture ; c, atrium ; el, elaeoblast ; es, endostyle ; g, gill ; h, 

 heart ; fis, haemal side of the bud ; i, intestine ; Ib, left half of the branchial sac ; 

 nt, opening of the oesophagus; n, nervous system; ns, neural side of the bud; o, 

 ovary ; rb, right half of the branchial sac ; s, stomach. 



