154 CCELENTERA. 



It is certainly suggestive that we have jelly-fish wholly free (Pelagia), 

 jelly-fish with a sedentary larval life (Awelia), jelly-fish predominantly 

 passive (Luceriiaria), and related polypes (Sea-anemones, etc.), which 

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

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

 liberated from (Campanularian and 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. It must be noticed 

 that the most frequent larval form is the planula, so that, if we regard 

 the gastrula as the ancestral type, the life history is not here a recapitu- 

 lation of the race history. 



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 

 early life is parasitic on sturgeons' eggs, the compound 

 Cordylophora, occurring in canals and in brackish water, and 

 the fresh-water Medusoid (Limnocodiuni) found in a tank in 

 the Regent's Park Botanic Gardens, and another similar form 

 recently discovered in Africa. Most of the active swimmers 

 are pelagic, but there are also a few active forms in deep 

 water. Many polypes anchor upon the shells of other 

 animals, which they sometimes mask, and there are most 

 interesting constant partnerships between hermit-crabs and 

 sea-anemones, e.g. between Pagurus prideauxii and Adamsia 

 palliata. 



The hermit-crab is masked by the sea-anemone, and may 

 be protected by its stinging powers ; the sea-anemone is 

 carried about by the hermit-crab, and may get crumbs from 

 its abundantly supplied table. This illustrates a mutually 

 beneficial partnership or commensalism, which, however, in 

 some other animals may degenerate into parasitism. (See 

 Fig. 1 6). 



