PYROSOMA STRUCTURE, DEVELOPMENT 93 



languets. The nerve-ganglion (on which 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 {l.o), 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 {cT), is 

 shown in Fig. 57. The heart (Ht) lies 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, 

 imnrediately behind the stolon, with part of which it is continuous, 

 a portion of this germinal tissue gives rise to a lobed testis {Us), 

 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 sm-face 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 



' "Oozooid" 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. 



