36 



ZOOLOGY 



SECT. 



slom 



ebl 



FIG. 695. Late stage in the development of Salpa, showing 

 the placental connection with the parent, atr. KJ>. atrial 

 aperture ; br. branchia ; cil. yr. ciliated groove ; tbl. elseo- 

 blast ; end. endostyle ; n. gn. nerve-ganglion ; *. oesopha- 

 gus ; or. ap. oral aperture ; jwric. pericardium ; pi. placenta ; 

 rect. rectum ; xtol. stolon ; stom. stomach. (From Korschelt 

 and Heider, after Salensky.) 



stance that during segmentation there is a migration inwards of 

 some of the cells of the follicle and of the wall of the oviduct, which 

 enter the segmenting ovum arid pass among the blastomeres. 

 There is uncertainty as to what part these inwardly-migrating 



cells play in the de- 

 velopment of the 

 embryo. According 

 to one observer they 



rfxtf$S8l ^m*^nMtV*&>> act merely as car- 



riers of nourishment, 

 and become broken 

 up and eventually 

 completely absorb- 

 ed ; according to 

 another they actu- 

 ally displace the 

 blastomeres and give 

 rise to the greater 

 part of the embryo. 

 There is no tailed 

 larval stage, and the 

 embryo develops the 

 muscle-bands and 

 all the characteristic 

 parts of the adult 



while still enclosed within the body of the parent and nourished by 

 means of the placenta. This sexually-developed embryo, however, 

 does not give rise to a form exactly like the parent, but to one 

 which differs from the latter in certain less important features and 

 notably in the absence of reproductive organs. The sexually 

 formed embryo, in other words, forms an asexual generation. It- 

 escapes to the exterior and becomes free-swimming (Fig. 684). 

 After a time there is developed a process or stolon (stol.), on the 

 surface of which are formed a number of bud-like projections. 

 These increase in size as the stolon elongates and each eventually 

 assumes the form of a sexual Salpa. The stolon with the Salpae 

 attached becomes separated off and swims about as a chain of 

 zooids in which the reproductive organs become developed. 



Distribution, etc. The pelagic forms are, as is the case with 

 most pelagic organisms, of very wide distribution, and none of the 

 genera are confined to particular oceanic areas. The fixed forms, 

 both simple and composite, are also of world-wide distribution ; 

 they are much more abundant in the southern hemisphere than in 

 the northern the composite forms attaining their maximum in 

 the South Pacific area. The depth to which the pelagic forms 

 extend has not been determined. Fixed forms occur at all depths, 

 but are much more numerous in shallow water than in deep, and 



