IV 



CCELENTERATA 



159 



branches are little cup-like projections with radiating processes 

 passing from the wall of the cup towards the centre, and thus 

 closely resembling the true cup-corals belonging to the Actinozoa 

 (vide p. 202). But in the * case of Stylaster each " cup " is 

 the locus, not of one, but of several zooids a polype projecting 

 fronxjts centre, and a dactylozooid from each of the compartments 

 of its peripheral portion. A calcareous projection, the style, the 

 presence of which is the origin of the generic name, rises up from 

 the tabula at the bottom of each cup. 



The gonophores in most species of Millepora are developed in 

 certain of the pores in dilatations or ampulla ; in one species at 

 the apices of the 

 d a c t yl o zooids. 

 They are me- 

 dusae, but never 

 have the com- 

 plete medusa- 

 form, being de- 

 void of velum, 

 mouth, radial 

 canals and ten- 

 t a c 1 e s. Both 

 male and female 

 medusae become 

 free, but the 

 period of free 

 existence is very 

 short. 



In Stylaster the 

 medusofd charac- 

 ter is much more 

 completely lost, 



and the gonophores are more of the nature of sporosacs or de-' 

 graded reproductive zooids lodged in special chambers (a) of the 

 coral. 



The Hydrocorallina occur only in the tropical portions of the 

 Pacific and Indian Oceans, where they are found on the coral- 

 reefs partly or entirely surrounding many of the islands in 

 those seas. Fossil forms are found as far back as the Triassic 

 epoch. 



ORDER 4. SIPHONOPHORA. 



The diversity of form exhibited by the members of this order is 

 so great that anything like a general account of it would only be 

 confusing to the beginner, and the most satisfactory method of 

 presentation will be by the study of a few typical genera. 



Halistemma (Fig. 120, A) occurs in the Mediterranean and other 



FIQ. 119. Stylaster sanguineus. A, portion of skeleton, 

 natural size ; B, small portion, magnified ; a. ampullae ; d.p. 

 dactylopores ; g.p. gastropores. (After Nicholson and Lydekker.) 



