ECOLOGY OF CCELENTERA 209 



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

 animal, without any youthful free-swimming stage. 



Ecology. — The Coelentera are almost all marine. In 

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

 hydra without tentacles, the strange Poly podium, which in 

 early life is parasitic on sturgeons' eggs, the compound 

 Cordylophora, occurring in canals and in brackish water, 

 and the fresh-water Medusoids {Limnocodium and 

 Limnocnida). Most of the active swimmers are pelagic, but 

 there are also a few active forms in deep water. Many 

 polyps anchor upon the shells of other animals, which they 

 sometimes mask, and there are m.ost interesting constant 

 partnerships between hermit-crabs and sea-anemones, e.g. 

 between Eupagurus prideauxii and Adamsia palhata. 



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 external partnership or commensalism. In some 

 other animals it may degenerate into parasitism (see Fig. 



Another kind of partnership is illustrated by many sea- 

 anemones and Alcyonarians. Minute unicellular Algae 

 (Zoochlorellae) live within the cells of the animals in close 

 physiological partnership with them (symbiosis). 



A spatial partnership in which one animal finds habitual shelter within 

 or near another is not infrequent; e.g. small horse-mackerels (Carangida;) 

 swimming in shelter of large jelly-fish; a small fish [Amphiprion 

 bicinctus) inside a giant sea-anemone (Crambactis arahica) which has 

 a diameter of a foot ; another fish (Fierasfer) thai goes in and out of 

 the hind-gut of Holothurians ; another that lives among the very long 

 hair-like spines of the Red Sea rock-urchin {Diadema saxatile) ; and 

 another {Apogonichthys strombi) that spends part of its time in the 

 mantle cavity of the large sea-snail {Strombus gigas) of the Bahamas. 



The quaint little hydroid Lar sabellarum^Yives at the mouth of the 

 tubes of the worm Sabella ; another hydroid {StylacHs minoi) grows 

 all over the skin of a rock-perch [Minotis) from the Indian Ocean ; 

 Stylactis vermicola was found on the back of the worm Aphrodite at 

 the great depth of 2900 fathoms. 



M 



