140 



THE INVERTEBRATA 



the general surface of the ectoderm. "Eyes" are however a general 

 character of the Anthomedusa as " Ears " (as statocysts may be broadly 

 termed) are of the Leptomedusa. 



Among the hydroids with sessile medusoids or gonophores there are 

 many forms in which the medusoid structure is lost, and a bud-like 

 structure is found in which a transverse section shows simply succes- 

 sive layers of ectoderm with generative cells, structureless lamella and 

 endoderm round an enteron which does not open by a mouth. In 

 forms like this the migration of germ cells, mentioned as occurring in 

 Obelia, are very noticeable. Thus in Eudendrium (Fig. 119 D) the 

 germ cells are often to be distinguished making their way along the 

 coenosarc towards developing gonophores. If this degeneration of 

 medusae is followed to its conclusion, a stage is arrived at in which 



Fig. 118. Margellium, example of an anthomedusan. M. mouth; man. manu- 

 brium; i. testis; ten. or. oral and ten.m. marginal tentacles. Original. 



there are no special reproductive buds at all, but the generative cells 

 occur in the body of the hydroid. This is the condition in Hydra, 

 where the multiplication of the interstitial cells at different positions 

 produces testes or ovaries. In the latter case each ovary contains a 

 single egg of a size unusual in the Hydrozoa, which grows by the 

 ingestion of its sister oocytes and the conversion of their proto- 

 plasm into yolk spherules. This phenomenon appears to be a con- 

 sequence of the habitat of the genus. As in so many other freshwater 

 animals, a free-swimming stage is omitted from the early history and 

 the period of larval development is passed in the shelter of the egg 

 shell; when the gastrula stage has been arrived at and the yolk is 

 mostly absorbed, development is suspended during a resting stage of 

 three or four weeks. After this the young Hydra pokes its oral end 



