LIFE-HISTORY OF SALPS 



5" 



The one set includes the free-swimming genera Salpa and 

 Doliolmn, together with the aberrant deep-water genus 

 Octacnemus ; the other, a few active free-swimming forms, 

 which exhibit throughout Hfe many of the characteristics of 

 the larval Ascidian. Of these, Appendicular ia is the most 

 familiar type. 



Both Salpa and Doliolum are pelagic in habit, and differ markedly in 

 structure from the Ascidians. The body is fusiform (Salpa) or barrel- 

 shaped {Doliolum), and wholly or partially encircled by definite muscle 

 bands, which replace the scattered fibres of the Ascidians. The mouth 

 is at one end of the body, and the atrial aperture at the other ; the 

 animals swim bv forcing the water out of the peribranchial chamber 



a.a 



PPb ph s.ns 



Fig. 293. — Diagram of Salpa africana. 



o.a., Oral aperture ; d.t., dorsal tubercle ; te., tentacle ; g., ganglion ; m., 

 muscle bands ; atr., atrium ; b.v., blood vessel ; an., anus ; a.a., 

 exhalant aperture ; v.n., visceral nucleus ; h., heart ; St., stolon ; d.L, 

 dorsal lamina ; E., endostyle ; s.n.g., sub-neural gland ; ph., pharynx ; 

 p.p.b., peri-pharyngeal band. 



posteriorly. Many of the most marked signs of specialisation in the 

 Ascidians are here absent. Thus the test may be, as in Doliolum, very 

 thin and devoid of cells, and the branchial sac is relatively simple in 

 structure ; the ciha on its walls are never so important in producing the 

 respiratory current as in the Ascidians, and the gill-slits may be few in 

 number, or, as in Salpa, may be represented by two large holes in the 

 walls of the pharynx. Further, the hermaphroditism is modified by the 

 occurrence of very marked protogyny, and the ova are never numerous 

 — in Salpa each sexual individual usually produces only one. 



On the other hand, the development exhibits marked alternation of 

 generations, both solitary and colonial forms being included in one life- 

 history. 



In Doliolum the fertilised egg gives rise to a tailed larva, which 

 develops into an asexual " nurse," possessing the power of budding (of. 



