STRUCTURE AND CLASSIFICATION 17 



explained. The tentacles are organs for catching and in 

 some cases killing or paralysing the food which passes within 

 their reach in the surrounding water, and the food is passed 

 through the mouth into a cavity where it is digested. The 

 food consists of various floating or drifting micro-organisms, 

 mostlv animal in nature, so that this method of feeding is 

 similar to that of other animals or, as it is called in technical 

 language, holozoic.^ 



The polyps also possess the power of mov^ement. It is 

 true that they cannot move from place to place in search 

 of their food as the higher animals do, but they are provided 

 with bands of muscles which enable them to expand and 

 retract their bodies. They are sensitive and irritable, 

 responding by muscular movements to stimuli of light, 

 heat, and chemical change in the surroundings. 



They produce in a season of the year eggs and sperms, 

 and the eggs when fertilised give rise to ciliated larvae 

 which swim aw^ay and develop into a new polyp or colony 

 of polyps. All these characters, combined with features of 

 more minute structural anatomy which it is not necessary to 

 describe in detail, prove that the polyps are solely and com- 

 pletely animal in nature. 



Some of the polyp-bearing corals possess an additional 

 character which Linnaeus considered to be also an attri- 

 bute of animal life only, but which we now know^ may 

 also occur in plants, that is, the secretion of calcium 

 carbonate. 



The calcium carbonate is secreted in various ways in 

 different kinds of corals, but there is this in common to all 

 of them, that it is always secreted by cells of the outer layer 

 of the body — the ectoderm — and is therefore, strictly speak- 

 ing, an outside support or exoskeleton, although in some 

 corals it becomes deep-seated and internal by subsequent 

 changes in its relation to the soft parts. 



The calcium carbonate which is secreted by the ectoderm 

 cells solidifies to form the complex calcareous structures of 

 such varied shape and structure with which we are famnliar 

 in our museum collections as the " Corals." The word 



^ For a further note on the nutrition of corals, see p. 20. 



