632 



INVERTEBBATE MORPEOLOGT. 



buds, as in BotryUus. The buds arise from the branchial 

 sacs behind the endostyle, and, on separating from the parents, 

 place themselves between them and the opening of the com- 

 • mon cloaca, so that the oldest members of the colony lie at 

 the closed end of the cylinder. On each side of the branchial 

 sac of each individual near the anterior end, or more precisely 

 near the peripharyngeal ciliated bands, is a mass of cells 

 which are brightly phosphorescent, the entire colony, which 

 may reach a length of over a metre, emitting a brilliant light 

 when stimulated. 



The development of Pyrosoma is exceedingly interesting 

 inasmuch as it presents an alternation of generations. From 



A B 



Fig. 289.— Larval BtroDrNG of Pyrosoma. A, embryo divided into the 

 cyathozooid aDd four ascidiozooids; B, later stage showing the ascidiozo- 

 oids twisting to form the circle of four primary individuals (after Kowa- 

 lewsky). 



cl = cloaca. en = endostyle. 



el = elseoblast. h = heart. 



n = nerve-ganglion of ascidiozooid. 



the embryo which develops from the egg at a very early 

 stage a stolon develops (Fig. 289), containing a prolongation 

 of what corresponds to the embryonic branchial sac and also 

 of the embryonic mesoderm. The embryo itself never 

 reaches a full development and is termed the Cyathozooid, 

 serving to supply the individuals developed from the stolon 

 with nourishment until they have reached a certain stage of 

 development. This it is able to do on account of the ovum 

 being plentifully supplied with yolk, which the Cyathozooid 

 gradually absorbs. The stolon at an early stage divides into 



