40 



ZOOLOGY 



SECT. 



ai-r-fr-p br 



slo 



reel 



ebL 



circumstance that during segmentation there is a migration in- 

 wards of some of the cells of the follicle and of the wall of the 

 oviduct, which enter the segmenting ovum and pass among the 

 blastomeres. There is uncertainty as to what part these inwardly 

 migrating cells play in the development of the * embryo ; but 

 probably they act merely as carriers of nourishment, and become 

 broken up and eventually completely absorbed. 



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, gives rise to 

 an asexual generation, 

 which escapes to the 

 exterior and becomes 

 free - swimming (Fig. 

 736). After a time 

 there is developed a 

 process or stolon;(sfoL), 

 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 Salpse attached becomes separated off and swims 

 about as a chain of zooids in which the reproductive organs are 

 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 

 at great depths are comparatively poorly represented, the simple 

 forms extending to a greater depth than the composite. Several 



FIG. 749. Late stage in the development of Salpa, showing 

 the placental connection with the parent, atr. ap. atrial 

 aperture ; br. branchia ; oil. gr. ciliated groove ; ebL elseo- 

 blast (mass of tissue probably representing a vestige of the 

 tail) ; end. endostyle ; n. gn. nerve-ganglion ; ces. oesopha- 

 gus ; or. ap. oral aperture ; peric. pericardium ; pi. 

 placenta ; rect. rectum ; stol. stolon ; stom. stomach. 

 (From Korschelt and Heider, after Salensky.) 



