l62 THE INVERTEBRATA 



occurring in Obelia, are very noticeable. Thus in Eudendrium 

 (Fig. 122 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 there are no special reproductive buds at all, but 

 the generative cells occur in the body of the hydroid. This is the con- 

 dition 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 

 protoplasm into yolk spherules as in Tubularia. This phenomenon 

 appears to be a consequence 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 out of the shell and, after creeping about for a short time, 

 frees itself and develops a mouth and tentacles. Other characters which 

 differentiate /fyd/ra from the majority of hydroids are the solitary habit, 

 which it shares with some Gymnoblastea, the hollow tentacles and the 

 complete absence of a stiffperisarc, this enabling the animal to execute 

 its characteristic looping movements. It is often pointed out that the 

 presence of a distinct migratory phase, the medusa, would entail a 

 serious disadvantage on Hydra ; it is suggested that the medusae might 

 be swept out to sea and lost. Hydra usually lives in ponds and is there- 

 fore hardly subject to this danger, but at the same time the embryo 

 in its horny egg shell is admirably fitted for dispersal, for example in 

 mud on the feet of migratory birds. This modification of reproductive 

 habits in Hydra is paralleled in the freshwater sponges with their 

 gemmules, the freshwater polyzoa with their statoblasts and the 

 cladoceran Crustacea with their ephippial eggs. It must, however, be 

 mentioned that a remarkable group of freshwater medusae occur 

 which belong to the Trachylina, and a stage occurs in their life history 

 which has sometimes been compared with Hydra and named a 

 separate genus {Microhydra) of hydroid polyps. This is, however, an 

 interesting case of convergence. 



The following genera of Calyptoblastea may be shortly mentioned : 

 Plumularia (Fig. 122 A) with a creeping hydrorhiza, giving off 

 plume-like branches, each of which bears a series of hydrothecae on 

 one side only; hydrothecae small, so that the polyps cannot be com- 

 pletely retracted within them; beside the nutritive polyps a second 

 smaller kind (nematophore), without mouth, but with long amoeboid 



