COELENTERATA 



53 



This is the UastostyU (Fig. 3 8, 4), and it bears a series of buds 

 called gonophores. These buds increase in maturity as they 

 approach the end of the blastostyle, so that a single blastostyle 

 may illustrate the various stages in the development of the 

 buds. The gonophores or rudimentary medusoid persons arise 

 as elevations of the ectoderm, into which the endoderm pro- 

 jects. At the free end of the bud the ectoderm then thickens, 



Fig. 39. — Diagrams to show the relations of (I.) a Hydroid polype to (II.) a Hydro- 

 medusa and to (III.) a Soyphomedusa. 



1. Ectoderm, shaded. 



2. Endoderm, dotted. 



3. Structureless lamella, black line. 



4. Coelenteron. 



5. Extension of coelenteron into tentacles 



in I., and forming radial canals in 

 II. and III. 



6. In I., Mouth. 

 In II., Velum. 



7. Gelatinous tissue, cross-hatched. 



8. Circular canal. 



9. Tentacle into which the endoderm 

 extends. 



10. Generative organs. In II. they occupy 



Anthomedusan position. 



11. In II., Generative organs in Lepto- 



medusan position. 

 In III., Sub-genital pit. 



12. In II., Endodermal lamella. 

 Inlll., Eoldofumbrellaovertentacle. 



13. The gastric filaments, remains of the 



taeniolae. 



and in this thickened portion a split appears in such a way as to 

 separate a central portion, the manubrium, from a circular por- 

 tion which surrounds the former and forms the umbrella. The 

 space thus constituted, which is lined everywhere by ectoderm, 

 is the sub-umbrella cavity (Fig. 40). The endoderm projects both 

 into the manubrium and the umbrella ; in those forms which 

 break off as free medusae, the manubrium acquires a mouth, which 

 leads into a cavity — the coelenteron — lined by endoderm. The 

 endoderm of the umbrella in Tuhularia is a ilat plate of tissue 

 enclosing no cavity except round the rim, where slight remnants 

 of a circular canal can be found, and from this rudiments of 

 radial vessels lead to the cavity of the manubrium. Eudi- 



