THE HYDROMEDUSAE 



on the course of the coenosarcal canals, and often lies in a special 

 pit of the coenenchyme, termed the ampulla. An entocodon is not 

 formed in the usual way ; instead of this, which is an ectodermal 

 downgrowth to be hollowed out eventually into the subumbral 

 cavity, the body wall at the sides of the generative cells grows 

 upwards, and arches over the manubrium to form the same cavity 

 (Millepora). All traces of medusoid structure are lost in some 

 cases (male Distichopora). Radial canals may be entirely absent 



Fio. 43b. 



Fio. 43c. 



43b. Diagrams illustrating the successive stages in the development of the cyclosystems 

 of the Stylasteridae. 1, Sporadopora ; 2, 3, AUopora; 4, 5, Stylaster ; 6, Astylus subviridis; 7, 

 Distichopora coccinea. s, style ; d)>, dactylopore ; gp, gastropore ; b, in tig. t>, inner horseshoe- 

 shaped mouth of gastropore. (After Moseley, from Lankester.) 



43c. Portion of the corallum of Astylus subviridis (one of the Stylnsteridae), showing 

 cyclosystems placed at intervals on the branches, each with a central gastropore and zone of 

 slit-like dactylopores. (After Moseley, from Lankester.) 



(Millepora), or may be present in varying numbers (twelve in 

 female Distichopora). 



REPRODUCTION. Asexual gemmation of hydroids is apparently 

 of the usual laminar character. The development of the sexually- 

 produced embryo has not been traced. 



ORDER 6. Siphonophora. 



DEFINITION. Colonial free -swimming Hydromedusae with 

 numerous polymorphic modifications of both hydroid and 



