CCELENTERATA : HYDROZOA. 8$ 



first consist simply of a tubular prolongation of the ectoderm 

 and endoderm, enclosing a caecal diverticulum of the body- 

 cavity ; but a mouth and tentacles are soon developed, when 

 the new being is usually detached as a perfect independent 

 Hydra. The Hydrm thus produced throw out fresh buds, 

 often before they are detached from the parent organism, and 

 in this way reproduction is rapidly carried on. 



In the second or sexual mode of reproduction, ova and 

 spermatozoa are produced in outward processes of the body- 

 wall (fig. 12, b). The spermatozoa are developed in little 

 conical elevations, which are produced near the bases of the 

 tentacles, and the ova are enclosed in sacs of much greater 

 size, situated nearer the fixed or proximal extremity of the 

 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 fecundated 

 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 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 

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

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

 phores, zvhich 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 

 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 iti 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 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 " hydrothecse." 



