ASCIDIAE SALPAEFORMES. 



41 



ft 



budding stolon (st) is a projection of the ventral body wall and contains a 

 diverticulum of the pharynx which comes off just behind the endostyle, a 

 genital cord, and a prolongation of the pericardium which latter structure 

 is said not to give rise to any organ in the bud. The ganglion carries on 

 its ventral side a pigmented organ which is interpreted as an eye, and the 

 phosphorescent organs (Fig. 33) are paired (right and left) masses of cells 

 containing a fatty substance at the level of the peripharyngeal band in 

 the blood space there found. 



The development* of Pyrosoma is very remarkable. The ovum is large 

 and full of yolk. The cleavage is partial and gives rise to a disc of cells 

 lying upon the yolk. There is no tailed stage and the development is very 

 different from that of other Ascidians. It takes place within the egg- 

 follicle and leads to 

 the formation of an 

 imperfect individual 

 called by Huxley the 

 cyathozooid. This while 

 still in the maternal 

 tissues produces a sto- 

 lon (Fig. 34), which 

 immediately undergoes 

 imperfect transverse 

 fission into four parts. 

 Each of these develops 

 into a typical Ascidian 

 zooid, called by Hux- 

 ley the ascidiozooid 

 (Fig. 35). The four 



ascidiozooids arrange p la . 34. Diagram of a Pyrosoma embryo showing the devel- 



themselves equatori- P m g cyathozooid lying on the yolk with some of its organs 



developed, and the stolon st with commencing fission. 



ally round the parent ,/ enteron ; do yolk ; e atrial aperture ; g ganglion ; h heart ; 



cvathozooid (Fig 35) fi ciliated pit, of cyathozooid ; r edge of germinalfdisc grow- 



ing over the yolk of cyathozooid ; stfstolon. 



acquire a common 



cloaca and form the first individuals of a new colony. The cyathozooid 

 now disappears, the ascidiozooids lose their primary connection, and the 

 little colony passes into the atrium of the parent and thence to the ex- 

 terior. Each of the zooids composing it possesses a ventral stolon which 

 immediately begins to bud. As all the successively produced zooids have 

 ventral stolons, the colony constantly increases in size and number of 

 individuals until the limit of growth is reached. 



Budding by which the adult colony is formed, f Each of the f our primary 

 ascidiozooids possesses a stolon at the hind end of its endostyle. This stolon 

 is the pedicle which in an earlier stage connected the zooid to that next 

 it in the chain, i.e. it is a part of the original stolon which does not 

 develop into an ascidiozooid. It contains (Fig. 36) a diverticulum of 

 the pharynx (d), two lateral bands of mesoderm, an unpaired cord of 

 mesoderm called the genital cord (g) and the elaeoblast. Of these 

 structures the mesodermal bands break up and are said not to give 

 rise to organs, while the cord of genital mesoderm which is at first 



* Kowalevsky, Arch. f. Mik. Anat., 11, 1875, p. 597. 

 t Seeliger, Jen. Zeitsch., 23, 1889, p. 595. 



