CCELENTERATA : HYDROCORALLIN^. 



149 



ture which is wanting in the purely prehensile dactylozooids. 

 The true Hydrozoal character of these extraordinary organisms 

 is conclusively shown by the fact that the reproductive organs 



Fig. 66. A, Portion of the skeleton of Stylaster sanguineus, of the natural size; B, 

 Small portion of a branch of the same, enlarged, showing the calices and ampullae. 

 Living in the Australian Seas. (After Milne-Edwards and Haime.) 



are situated outside the bodies of the ordinary zooids, being in 

 the form of fixed sporosacs developed within sac-like cavities 

 (" ampullae") in the skeleton (fig. 66, B), which at certain 

 periods communicate with the exterior by minute pores. 



CHAPTER XI. 

 DISTRIBUTION OF THE HYDROZOA. 



I. DISTRIBUTION OF HYDROZOA IN SPACE. The genera of 

 Hydrozoa have a wide distribution, the mode of reproduction 

 amongst the fixed forms being such as to insure their extension 

 over considerable areas. The various species of Hydra are 

 of common occurrence in the fresh waters of various regions 

 of the world. Cordylophora, the sole remaining fresh -water 

 genus, has not been found to occur out of the north temperate 



