;to 



ZOOLOGY 



other cases a mere bud is formed which becomes full of sexual 

 products without ever acquiring resemblance to a jellyfish.' 

 The jellj'fish is the primitive tyi^e which has undergone a 

 reduction in some cases to a gonophore. It is an interesting 

 fact that in certain species sometimes jellyfishes and some- 



cndJam 



Fig. 2G0. — Di.ssrctioii of a medusa \\ith rather more than one-rjuarter of 

 the uml)rolla and manubrium eut awa>' (diasrammatie). The ectoderm is 

 dotted, tlie entoderm {end. Inm.), .striated: the hUiek is a membrane be- 

 tween them ; /, eyst containing a little sensory- l)od>' ; vl, "velum." a shelf 

 to diminish the opening of the Viell. Food .s_\-stem : mth, mouth: miib, 

 inanul>rium, ov gullet ; rad.c. n'r.c, radial and eireular eanals, parts of the 

 digesti^■e ra\-it>'. Reprodtietive system : gon, germ-cells. 



times gonoiihores will be produced. In the bell-hjxlroids the 

 gonophores are encased in a cuticular capsule, but in the 

 tuljularian hydroids thejr are quite naked. 



There are certain hydromedusse in which the hydroid stage 

 is unknown or known to be lacking. Here belong some jelly- 

 fishes of our Eastern coast, which may become twenty to thirty 

 centimetres in diameter (Fig. 261). Besides these the sea 



1 See Fig. 2.54, c 



