HYDROZOAN CORALS 147 



not always present ; in fact, specimens of the kinds shown in 

 Fig. 68 may be regarded as rarities in our collections, for, 

 unlike many other Hydrozoa, Millepora does not produce 

 its medusae continuously or over a long period of time, 

 but so far as we can judge only occasionally and then in 

 great profusion. But our knowledge of the periodicity of 

 medusa production in Millepora in any part of the world 

 is still lacking in precision. 



Passing now to the structure of the living tissues which 

 form the corallum we find that there are two kinds of 

 polyps — the gasterozooids and the dactylozooids — inhabiting 

 the gasteropores and dactylopores respectively. The gas- 

 terozooids are short and stumpy polyps projecting only a 

 little way above the surface of the corallum when fully 

 extended (Fig. 67, G.). They have a terminal mouth and a 

 digestive cavity, in which occasionalh^ a small crustacean 

 may be found as food, and round the mouth are four knobs, 

 armed with nematocysts, which probably represent four 

 rudimentary tentacles. 



The dactylozooids (Fig. 67, D.) when fully extended are 

 long, slender, hollow structures provided with a variable 

 number of short capitate tentacles arranged alternately or 

 more irregularly on the body wall. They have no mouths. 

 There can be no doubt that the function of the dactylozooids 

 is to catch and paralyse the small living organisms that 

 come within their reach and to pass them to the gasterozooids 

 to swallow and digest — an admirable example of efficient 

 division of labour. The zooids are connected together 

 beneath the surface by an elaborate system of branching and 

 anastomosing coenosarcal canals. These canals are pro- 

 vided with a double lining of cells. The outer layer of cells 

 — the ectoderm — is mainly concerned with the secretion of 

 the calcium carbonate that forms the corallum. The inner 

 layer of cells — the endoderm — may serve the purpose of 

 providing the ciliary action necessary for the maintenance 

 of the circulation of currents of water through the canals, 

 but on that point further investigation on living material is 

 needed. The most striking feature of the canal system is 

 the presence, in enormous numbers, of the symbiotic 



