Ill DOLIOLUM — STRUCTURE 97 



small stigmata ; consequently there is a free passage for the 

 water through the body along its long axis, and the animal 

 swims by contracting its ring-like muscle-bands so as to force out 

 the contained water posteriorly. When stigmata are found on 

 the lateral walls of the branchial sac (see Fig. 59) there are 

 corresponding anteriorly directed diverticula of the peribranchial 

 cavity. There is a distinct endostyle on the ventral edge of the 

 branchial sac and a peripharyngeal band surrounding its anterior 

 end, but there is no representative of the dorsal lamina along its 

 dorsal edge ; and there are neither branchial nor atrial tentacles. 

 The oesophagus commences rather on the ventral edge of the 

 posterior end of the branchial sac, and runs backwards to open 

 into the stomach, which is followed by a curved intestine opening 

 into the peribranchial cavity. The alimentary canal as a whole 

 is to the right of the middle line. The hermaphrodite repro- 

 ductive organs are to the left of the middle line alongside the 

 alimentary canal. They open into the peribranchial cavity. 

 The ovary is nearly spherical, while the testis is elongated, and 

 may be continued anteriorly for a long distance. The heart is 

 placed in the middle line ventrally, between the posterior end of 

 the endostyle and the oesophageal aperture. The nerve-ganglion 

 lies about the middle of the dorsal edge of the body, and gives 

 off many nerves. Under it is placed the neural gland, the 

 duct of which runs forward and opens into the anterior end of 

 the branchial sac by a simple aperture surrounded by the spirally 

 twisted dorsal ends of the peripharyngeal bands. 



Life -History. — The ova produced by the Doliolum of the 

 sexual generation, after a complete or " holoblastic " segmenta- 

 tion, and normal invagination, produce tailed larvae with a 

 relatively small caudal appendage, and a large body in which 

 the ■ characteristic musculature begins to appear (Fig. 60, A). 

 These larvae after metamorphosis lose their tails and develop 

 into oozooids, known as " nurses," which are asexual, and are 

 characterised (Fig. 60, B) by the possession of nine muscle-bands, 

 by the stigmata being few in number and confined to the posterior 

 end of the branchial sac, by an otocyst on the left side of the body, 

 by a, ventrally-placed complex stolon or " rosette organ " near the 

 heart, from which primary buds are produced by constriction, 

 and by a dorsal outgrowth (" the cadophore") near the posterior end 

 of the body. The buds (blastozooids) give rise eventually, after 



VOL. VII II 



