CHAP. XV Backboneless Animals 227 



phosphorescence, and their fuller life is associated with the develop- 

 ment of sense-organs and a more compacted nervous system. In 

 both sets the food usually consists of small organisms, in securing 

 which the tentacles and the stinging cells are of use. 



All the Stinging-animals are marine except the species of Hydra, 

 a minute relative called Microhydra, the hydroid Cordylophora which 

 occurs in brackish water and in canals, a strange form Polypodium 

 which is parasitic in its youth on the eggs of the Russian sturgeon 

 or sterlet, and a freshwater jellyfish (Limnocodium) which was 

 found in the tanks at Kew. The rest live in the sea. Hydroids 

 grow on rocks and shells and on the backs of crabs and other 

 animals which they mask ; sea -anemones live on the shore -rocks 

 — but not a few are found at considerable depths ; the medusoid 

 types frequent the opener sea where Siphonophores and Ctenophores 

 bear them company. 



Various kinds of corals should be contrasted. Dead-men's- 

 fingers with numerous jagged spicules of lime in its flesh is just 

 beginning to be coralline. Similar spicules have been fused together 

 in an external tube in the organ -pipe coral. In the red coral 

 the calcareous material forms an axis around which the individuals 

 are clustered. Very different are the reef-building corals, where 

 the cup in which each individual lived is more or less well marked 

 according as it has remained distinct or fused with its neighbours, 

 and where an image of the fleshy partitions of the sea- anemone- 

 like animal is seen in the radiating septa of lime. 



Corals are passive, and like many animals of similar habit have 

 calcareous shells, but how do they get the carbonate of lime of 

 which these are composed ? Is that salt — by no means abundant in 

 sea-water — plentiful near coral-reefs, or is there a double-decomposi- 

 tion between the abundant calcium sulphate and the coral's waste- 

 products, as has been suggested by Irvine and Murr&y ? On what 

 do the corals feed, for they seem always to be empty ? Do their 

 bright pigments enable them, as Hickson suggests, to feed like 

 plants on carbonic acid ? 



The struggle for standing-room should also be thought of, and 

 the throngs of gaily-coloured animals which browse and hide on 

 the coral banks. 



Many of the Stinging-animals have forms and colours which 

 delight our eyes, and the quaint partnerships between sea-anemones 

 and crabs are interesting. 



But it is through corals that Coelenterates come into closest touch 

 with human life. For the stinging of bathers by jellyfish is a 

 minor matter, and the thousands which are cast upon the beach 

 are of no use as manure, being little more than animated sea- 

 water. 



