CCELENTERATA I HYDROZOA. 



93 



by the spermatozoa, which are simultaneously liberated. The 

 embryo appears as a minute, free-swimming, ciliated body. The 

 serous and mucous layers of the blastoderm (germinal area) 

 correspond to the ectoderm and endoderm, and for the forma- 

 tion of the perfect Hydra nothing further seems wanting than 

 the modification of one end of the body into a hydrorhiza, and 

 the formation of a mduth and tentacles at the other. 



ORDER II. CORYNIDA ( = TUBULARIDA, the Athecata of 

 Hincks). The order Corynida comprises those Hydrozoa 

 whose hydrosoma is fixed by a hydrorhiza, and consists either of 

 a single polypite, or of several united by a ccenosarc, which usually 

 develops a firm outer layer or "polypary" No " hydrothecce " 

 are present. " The reproductive organs are in the form of gono- 

 phores, which vary much in structure, and arise from the sides of 

 the polypites, from the coenosarc, or from gonobla stidia " (Greene). 



The hydrosoma of the Corynida may consist of a single 

 polypite, as in Coryomorpha and Vorticlava, or it may be com- 

 posed of several united by a ccenosarc, as in Cordylophora 

 (fig. 19, a). The order is entirely confined to the sea, with 

 the single exception of Cordylophora, which inhabits fresh water. 

 In Tubularia and its allies the organism is protected by a well- 

 developed external chitinous envelope or "polypary;" but in 

 the other genera belonging to the order the polypary is either 

 rudimentary or is entirely absent. The polypary of the 

 Corynida, when present, is readily distinguished from that of 

 the Sertularida, by the fact that in the former it extends only 



Fig. 19. Morphology of Corynida. a Fragment of Cordylophora lacustris, slightly 

 enlarged ; b Fragment of the same considerably enlarged, showing a polypite and 

 three gonophores in different stages of growth, the largest containing ova ; c Portion 

 of Syncoryne Sarsii with medusiform zooids budding from between the tentacles. 



to the base of the polypites ; whereas in the latter it expands 

 to form little cups for the reception of the polypites, these cups 

 being called " hydrothecae." 



