344 ZOOLOGY sect. 



(i.e. the collection of the nitrogenous waste-matters) being appar- 

 ently carried on by the leucocytes and the cells of the funicular 

 tissue. 



Reproductive Organs. — Ovary and testis are found to occur 

 together in the same zooid. They are both formed from specially 

 modified cells of the parenchyma, either of the funiculus or of the 

 body-wall. The testis, developed from the cells of the funicular 

 tissue, gives origin to spherical masses of cells — the spermatidia 

 (sp) — which develop into sperms with very long motile tails. These 

 become free from one another and move about in the body-cavity 

 or in its prolongations into the tentacles. There is no spermiduct, 

 and it is doubtful if the sperms pass to the exterior. The ovary 

 (ov) is a small rounded body formed from the parietal layer of the 

 parenchyma about the middle of the zocecium ; it consists of only 

 a small number of cells of which only one at a time becomes a 

 mature ovum, certain smaller cells forming an enclosing follicle. 

 The mature ovum is perhaps fertilised in the ccelome ; it passes 

 into the interior of a rounded outgrowth of the zocecium — the 

 ocecium (ocec) — lined with parenchyma, and forming a sort of brood- 

 pouch in which it undergoes development. 



Development. — Segmentation (Fig. 277) is complete and 

 nearly regular. A blastula is formed having the shape of a 

 bi-convex lens. In the interior of the blastocoele or cavity 

 of the blastula, four cells (end) — the primitive endoderm 

 cells — become distinguishable : these increase in number by 

 division, and form a mass of free cells which almost completely 

 fill the blastocoele ; this mass apparently represents both 

 endoderm and mesoderm. Small cavities which appear in it 

 subsequently unite together to form the primitive ccelome. 

 A very broad ring-shaped thickening — the corona (G, cor.) — 

 is formed round the equator of the embryo and becomes 

 provided with cilia. A circular pallial groove arises on the 

 oral side of the corona. A sac-like, afterwards beaker-shaped 

 invagination of the ectoderm on what is destined to become 

 the oral side of the ciliated ridge, forms a larval structure, termed 

 the sucker (Fig. 278, suck), which afterwards serves to fix the larva. 

 A second depression of the ectoderm in the region of the corona 

 on the oral side forms the ectodermal groove. At the aboral pole is 

 developed, also from the ectoderm, a second larval structure — the 

 calotte or retractile disc (disc), on which motionless sensory cilia 

 appear. In close relation to the ectodermal groove is formed a 

 mass of cells, the pyriform organ (p). 



An alimentary canal is absent in the larva of Bugula when it 

 escapes from the ooecium. After an interval of free existence as a 

 ciliated larva, certain changes appear which lead to a very 

 complete metamorphosis. The sucker becomes everted by a 

 strong contraction of the body, and fixes the larva to some foreign 



