III PYROSOMA—STRUCTURE, DEVELOPMENT 93 
languets. The nerve-ganglion (onwhich is placed a small pigmented 
sense-organ, the unpaired “eye”), the neural gland, the dorsal 
tubercle, the peripharyngeal bands and the endostyle are placed 
in the usual positions. On each side of the anterior end of the 
branchial sac, close to the peripharyngeal bands is a mass of 
rounded mesodermal gland-cells (/.0), which are the source of 
the phosphorescence. They are apparently modified leucocytes 
lying in blood-sinuses. The alimentary canal is placed posteriorly 
to the branchial sac, and the anus opens into a large peribranchial 
or atrial cavity, of which only the median posterior part (c/), is 
shown in Fig. 57. The heart (//¢) les between the posterior 
end of the branchial sac and the intestine, close to where the 
endostyle is prolonged outwards to form the inner tube of the 
ventral stolon. The reproductive organs are developed from a cord 
of germinal tissue which forms a part of every budding stolon, and 
so establishes a continuity of origin between the ova of successive 
generations of Pyrosoma. On the ventral edge of the body, 
immediately behind the stolon, with part of which it is continuous, 
a portion of this germinal tissue gives rise to a lobed testis (¢es), 
and to a single ovum surrounded by indifferent or follicle-cells. 
Development and Life-History.—The development takes 
place within the body of the parent, in a part of the peribranchial 
cavity. It is a “ direct” development, the tailed larval stage being 
omitted. The segmentation is incomplete or “ meroblastic,” and 
an elongated embryo is formed on the surface of a mass of food- 
yolk. Follicle-cells, or kalymmocytes, migrate into the embryo, 
where they aid in its nutrition. The embryo (or young oozooid),! 
after the formation of an alimentary cavity, a tubular nervous 
system, and a pair of laterally placed atrial tubes, divides into 
an anterior and a posterior part (see Fig. 58). The anterior 
and ventral part, or stolon, then segments into four pieces (the 
tetrazooids or first blastozooids),’ which afterwards develop into 
the first ascidiozooids of the colony, while the posterior part 
remains in a rudimentary condition, and is what was called by 
Huxley the “cyathozooid” (Fig. 58, cy). This is really the 
degenerate oozooid, and eventually atrophies without having 
1 “ Qozooid”’ and ‘‘ blastozooid’’ have not always been used in the same sense. 
It is best to regard as oozooid the first member of a new colony derived from 
an embryo formed by the fertilisation of an ovum, and to call the remaining 
ascidiozooids produced by gemmation the blastozooids. 
