PHYLUM CHORDATA 



41 



o7:cip 



order: — (1) sexual form; (2) tailed larva developed sexually 

 from (I) ; (3) first asexual form or "nurse," the direct outcome of 

 (2); (4) second asexual form (phorozooid) developed on the 

 cadophore of (o) from buds originating on the ventral stolon ; (5) 

 the young of the sexual form (1) which are develoijcd on the 

 stalk of (4). 



Snlpa, like Doliolum, presents a remarkable alternation of 

 generations. In the sexual form, which has already been descriljed, 

 only one ovum is developed. The testis becomes mature later 

 than the ovum, and the latter is impregnated by .sperms from 

 the testis of an individual of an older chain. The development is 

 direct and takes place within the body of the parent, the embryo 

 as it grows ^project- 

 ing into the branchial "'"''^ """ 

 cavity. The nourish- 

 ment of the develoiJ- 

 ing embryo (Fig. 740) 

 is effected by the 

 formation of a struc- 

 ture — the placenta — 

 through which a close 

 union is brought 

 about between the 

 vascular system of 

 the parent and that 

 of the embryo. The 

 placenta of Salpa is 

 partly formed from 

 follicle-cells and ecto- 

 derm cells of the 

 embryo, jiartly from 

 the celLs of the wall 

 of the oviduct. Segmentation is complete. The study of the 

 earlier stages is complicated by the very remarkable and unusual 

 circumstance that during segmentation thei'e 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 wliile 

 still enclosed within the body of the pai-ent and nouiished 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 lattei' in certain less important features and 

 notably in the absence of reproductive organs. The sexually 



Fig. T-JO. — Late stage in the development of Salpa, sh-iwin^' 

 the placental connection with the parent, air. ■'/'. atoal 

 aperture; /jr. hranchia ; ctf. <n\ ciliated j^^n>o\-L' ; ./-/. el. co- 

 blast (maps of tissue probably representing' a vestiyc of the 

 tail); l'KI. endostyle ; n. ria. ncrvC't,';iii^Iion ; '..s. oesopha- 

 gus ; or. ap. oral aperture ; 'j>eri<: peneardiuni ; jil. placenta ; 

 rccL rectum ; s/ol. stolon ; .^(om. stomach, (l-'rorn Korschclt 

 and Hcidcr, after Salensky.) 



