BIONOMICS. 



167 



only occasionally rise into free activity ; while in the other series \ve 

 have medusoid types always free (Trachymedusas), others which are 

 liberated from (Campanularian and 1 Tubularian) sedentary hydroids, 

 other (Sertularian and Plumularian) zoophytes whose buds though often 

 medusoid -like are not set free, and finally Hydra, which, though it 

 may creep on its side, or walk on its head, is predominantly a sedentary 

 animal, without any youthful free-swimming stage. 



Bionomics.- -The Ccelentera are almost all marine. In 

 fresh water we find the common Hydra, the minute Micro- 

 hydra without tentacles, the strange Polypodium^ which in 



FIG. 81. Commensalism of sea-anemones and hermit-crab. 



early life is parasitic on sturgeons' eggs, the compound 

 Cordylophora^ occurring in canals and in brackish water, and 

 the fresh-water Medusoid (Limnocodimti). Most of the 

 active swimmers are pelagic, but there are also a few active 



