142 THE INVERTEBRATA 



out of the shell and, after creeping about for a short time, frees itself 

 and develops a mouth and tentacles. Other characters which differ- 

 entiate Hydra from the majority of hydroids are the solitary habit, 

 which it shares with some Gymnoblastea, and the complete absence 

 of a stiff perisarc, 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 dis- 

 advantage on Hydra ; it is suggested that the medusae might be swept 

 out to sea and lost. Hydra usually lives in ponds and is therefore 

 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. 119 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 

 processes which engulf decaying polyps and larvae of other sessile 

 forms. 



Sertularia (Fig. 119 B) with a creeping hydrorhiza, more or less 

 branching stems which bear opposite hydrothecae; hydrothecae 

 large, so that the polyps can completely retract within them. 



The following genera of Gymnoblastea may also be mentioned : 



Cordylophora, living in fresh or brackish water (Norfolk Broads), 

 polyps with scattered filiform tentacles. 



Pennaria (Fig. 119 C) with two kinds of tentacles, oral capitate 

 and aboral filiform; nematocysts of very large size. 



Hydr actinia, with spreading plate-like perisarc covered by naked 

 coenosarc, very often found coating a shell inhabited by a hermit crab ; 

 with spiral dactylozooids and sessile gonophores. 



Podocoryne, as Hydr actinia ^ but with free medusae. 



The polyp forms of many medusae, both Antho- and Leptomedusae, 

 are unknown. 



