XII 



BACKBONELESS ANIMALS 



231 



codium from Lake Tanganyika and elsewhere. Apart 

 from these and a few other exceptional forms, Coelentera 

 live in the sea. Hydroids grow on rocks and shells and 

 on the backs of crabs and other animals which they mask ; 



FIG. 66. A PORTION OF THE PRECIOUS CORAL OR RED CORAL OF COMMERCE, 

 Corallium riibrum, A COLONY OF ALCYONARIAN POLYPS. 

 (After Lacaze-Duthiers.) 



The polyps (C) or members of the colony are white. Each has eight 

 feathery tentacles surrounding the mouth; the general cavity (ccel- 

 enteron) between the gullet and the body wall is divided into eight com- 

 partments by eight upright partitions or mesenteries. 



The polyps are connected by red flesh (B) in which are imbedded canals 

 connecting the cavities of adjacent polyps with one another. The red- 

 ness is due to innumerable tuberculated spicules (of carbonate of lime) 

 of a red colour. 



In the centre there is the red stony axis, which is used in making orna- 

 ments. It is due to a coalescence of innumerable spicules, but how the 

 fusion is effected we do not know. 



sea-anemones live on the shore-rocks but not a few are 

 found at considerable depths ; the medusoids and jelly- 

 fishes frequent the opener sea, where Siphonophores and 

 Ctenophores bear them company. 



