xm 



PHYLUM CHORDATA 



exclusive function being by its contractions to propel the colony 

 through, the water. 



The zooids of the cadophofe consist of two sets, differing from 

 one another in position and in future history the lateral zooids 

 and the median zooids. The lateral zooids serve solely to carry on 

 the nourishment and respiration of the colony, and do not undergo 

 any further development. Some of the median buds, on the other 

 hand, become detached and take on the special character of phoro- 

 zooids. When free, each phorozooid 

 carries with it the stalk by means of 

 which it was attached to the stolon ; 

 on this stalk there have previously 

 become attached a number of buds 

 which are destined after a time to be 

 developed into the sexual zooids. 



The succession of stages in the 

 life-history of Doliolum thus briefly 

 sketched will be seen to succeed one 

 another in the following order : (1) 

 sexual form ; (2) tailed larva de- 

 veloped sexually from (1) ; (3) first 

 asexual form -or " nurse," the direct 

 outcome of (2) ; (4) second asexual 

 form (phorozooid) developed on the 

 cadophore of (3) from buds originating 

 on the ventral stolon ; (5) the young 

 of the sexual form (1) which are 

 developed on the stalk of (4). 



Salpa, like Doliolum, presents a 

 remarkable alternation of generations. 

 In the sexual form, which has already 

 been described, 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 

 projecting into the branchial cavity. The nourishment of the 

 developing embryo (Fig. 749) is effected by the formation 

 of a structure 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 ectoderm- cells of the embryo, partly from the cells of the 

 wall ofj}he oviduct. Segmentation is complete. The study of the 

 earlier stages is complicated by the very remarkable and unusual 



ctors.slol 



lal.bds 



mcd.bds 



FIG. 748. Doliolum, dorsal view of 

 the posterior part of the body of an 

 asexual /ooid, showing the course 

 taken by the buds (emb.) over the sur- 

 face from the ventral stolon (vent, 

 stol.) to the cadophore (dors, stol.) and 

 their growth on the latter, lat. bds. 

 lateral buds ; med. bds. median buds ; 

 peric. pericardium. (After Barrois.) 



