IV 



H YLUM CCELENTERATA 



157 



of carbonate of lime (corallum), the whole surface beset with the 

 numerous minute pores to which the genus owes its name. The 

 pores are of two sizes : the larger are about 1 or 2 mm. apart, and 

 are called gastropores (B, g.p) ; the smaller are arranged more or 

 less irregularly round the gastropores, and are called dactylopores 

 (d.p). The whole surface of the coral between the pores has a pitted 

 appearance. Sections (C) show that the entire stony mass is 

 traversed by a complex system of branched canals, which com- 

 municate with the exterior through the pores. The wide vertical 

 canals in immediate connection with the pores are traversed by 

 horizontal partitions, the tabulce (tb). 



Flu. 117. Millcpora alcicornis. 



magnified ; C, vertical section, magnified 

 tabulae. (After Nicholson and Lydekker.) 



A, part of skeleton, natural size ; B, portion of surface, 

 d.p. dactylopores ; g.p. gastropores ; tb. 



In the living animal each pore is the place of origin of a zooid : 

 from the gastropores protrude polypes (Fig. 118, G) with hypostome 

 and four knobbed tentacles ; from the dactylopores long, filamentous, 

 mouthless dactylozooids or feelers (D), with irregularly disposed 

 tentacles : the function of these latter is probably protective and 

 tactile, like that of the guard-polypes of Plumularia and the 

 dactylozooids of Hydractinia. The bases of the zooids are con- 

 nected with a system of delicate tubes, which ramify through the 

 canals of the coral and represent a much-branched coenosarc, 

 recalling that of Hydractinia (p. 146). 



The ccenosarcal tubes have the usual structure, consisting of 

 ectoderm and endoderm with an intervening mesogloea. From 

 the relative position of the parts it will be obvious that the calcare- 

 ous skeleton is in contact throughout with the ectoderm of the 



