COELENTERATA 53 
This is the blastostyle (Fig. 38, 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, 
EOS 
nee, 
nana 
rer, 
Fic. 39.—Diagrams to show the relations of (I.) a Hydroid polype to (II.) a Hydro- 
medusa and to (III.) a Seyphomedusa. 
1. Ectoderm, shaded. 9. Tentacle into which the endoderm 
2. Endoderm, dotted. extends. 
3. Structureless lamella, black line. 10. Generative organs. In II. they occupy 
4. Coelenteron. Anthomedusan position. 
5. Extension of coelenteron into tentacles 11. In II., Generative organs in Lepto- 
in I., and forming radial canals in medusan position. 
II. and III. In IIL., Sub-genital pit. 
6. In J., Mouth. 12. In II., Endodermal lamella. 
In IIL., Velum. InIII., Foldofumbrella over tentacle. 
7. Gelatinous tissue, cross-hatched. 13. The gastric filaments, remains of the 
8. Circular canal. 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 Tubularia is a flat 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.  Rudi- 
