iv PHYLUM 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 tabula (tb). 



. Y" .; 



A ,*%.': : - ff*~-9 * : ^ 



V : ^*-^ 



.;m '-v y f <*>*-* "& B 



* * * :> 



, . '-'' 



w 



^ '> 



FIG. 1 17. Millepora alcicornis. A, part of skeleton, natural size ; B, portion of surface, 

 magnified ; C, vertical section, magnified ; d.p. dactylopores ; g.p. gastropores ; W. 

 tabulae. (After Nicholson and Lydekker.) 



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 mesoglcea. ^rom 

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

 ous skeleton is in contact throughout with the ectoderm of the 



