HYDRIDA HYDROCORALLINAE. 127 



the gonangia. When the costae are well developed and the hydrothecae are 

 suppressed, the phylactocarp is called a corbula (Aglaophenia). 



The generative cells, though nearly always ripening in and discharg- 

 ing from the ectoderm of gonophores, make their first appearanpe in 

 a great variety of places : in the ectoderm of the free Medusa, in the 

 endoderm or ectoderm of the fixed gonophore, in the ectoderm of the 

 coenosark, near or remote from the budding gonophore, or even in 

 the endoderm. But wherever they arise they eventually migrate 

 passing, if necessary, through the structureless lamella to the gono- 

 phore, in the ectoderm of which they are generally, though not 

 always, contained. 



The Hydromedusae feed chiefly on animal substances, and Hydra 

 possesses chlorophyll bodies in its endoderm. Whether these are 

 chlorophyll bodies like those of plants or symbiotic Algae, like those 

 of Radiolaria, seems uncertain. The free-swimming Medusae and 

 Siphonophora are phosphorescent. With a few exceptions they are 

 marine organisms. 



Order 1. HYDRIDA. 



Solitary polyps without medusoid buds. Both generative products 

 are developed in the ectoderm of the polyp. 



In this order colonies are not formed, and there are no medusoid 

 individuals. The generative cells are produced by the ectoderm of 

 the polyp itself. There is only one genus, Hydra L., the fresh- 

 water polyp. 



The genera Protohydra Greeff, and Microhydra Potts, are possibly allied here. 

 Protohydra is marine, and reproduces by transverse fission. Microhydra is fresh- 

 water, and invested by a coat of mud. Both genera are without tentacles, and 

 the sexual reproduction is unknown. 



Order 2. HYDROCORALLINAE.* 



Colonial Hydromedusae consisting of a meshwork of coenosarcal 

 canals, the ectoderm of which secretes a hard calcareous matter filling 

 up the spaces of the meshwork. Polyps of two forms, gastrozooids 

 and dactylozooids. Gonophores of the form of rudimentary medusae 

 are generally present. 



For a long time the position of the Hydrocorallinae was uncertain. 

 L. Agassis first suggested that Millepora was not an Anthozoon but 

 a Hydroid, but it was not until Moseley had worked out the anatomy 



* H. N. Moseley, "Report on Corals," Challenger Reports, vol. 2. S. J. 

 Hickson, "The Medusae of Millepora Murrayi, etc," Q.J.M.S., vol. 32. 1891. 



