IV 



PHYLUM CCELENTERATA 



145 



fixes itself to the manubrium of one of the Trachymedusse which 

 serves as its host, and develops into a hydrula. But the latter, in- 

 stead of itself becoming metamorphosed into a medusa, retains the 

 polype form and produces other hydrulse by budding, these last 

 becoming converted into medusas in. the usual way. 



ORDER 3. HYDROCORALLINA. 



The best-known genus of Hydroid Corals is Millepora, one species 

 of which is* the beautiful Elk-horn Coral , M. alcicornis. The dried 

 colony (Fig. 103 A) consists of an irregular lobed or branched mass 



FIG. 103. Millepora alcicornis. A, part of skeleton, natural size; B, portion of surface, 

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

 (After Nicholson and Lydekker.) 



of carbonate of lime, 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 

 gcistroporcs (B, y.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 gastropores are traversed 

 by horizontal partitions, the tabulcc- (tb). 



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

 from the gastropores protrude polypes (Fig. 104, P) with hypostome 



