CCELENTERATA : HYDROZOA. 



8l 



animal. Ordinarily there is but one of these sacs containing 

 a single ovum, but sometimes there are two. When mature, 

 the ovum is expelled through the body-wall, and is fecun- 

 dated 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 

 formation of the perfect Hydra nothing further seems wanting 

 than the modification of one end of the body into a hydro- 

 rhiza, and the formation of a mouth 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 " hydrotheca" 

 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 ccenosarc, or from gonoblastidia" 

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



Fig. 13. 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 Syticoryne Sarsii with medusifbrm zooids budding from between the 

 tentacles. 



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 "poly- 

 pary;" but in the other genera belonging to the order the 

 polypary is either rudimentary, or is entirely absent. The 



VOL. I. F 



