CCELENTERATA I HYDROZOA. 71 



cylindrical hydrosoma is excavated into a single large cavity, 

 lined by the endoderm, and communicating with the exterior 

 by the 'mouth. This the 'somatic cavity' is the sole di- 

 gestive cavity with which the Hydra is provided, the in- 

 digestible portions of the food being rejected by the mouth. 



The Hydra possesses a most extraordinary power of resist- 

 ing mutilation and of multiplying artificially, when mechani- 

 cally divided. Into however many pieces a Hydra may be 

 divided, each and all of these will be developed gradually into 

 a new and perfect polypite. The remarkable experiments of 

 Trembley upon this subject are well known, and have been 

 often repeated, but space will not permit further notice of 

 them here. Reproduction is effected in the Hydra both 

 asexually by gemmation and sexually, the former process 

 being followed in summer, and the latter 'towards the com- 

 mencement of winter, few individuals surviving this season. 

 In the first method the Hydra throws out one or more buds, 

 generally from near its proximal extremity. These buds at 

 first consist simply of a tubular prolongation of the ecto- 

 derm 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 inde- 

 pendent Hydra. The Hydrce 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 sper- 

 matozoa are produced in outward processes of the body-wall 

 (fig-. 12 &). 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. Ordi- 

 narily there is but one of these sacs containing a single ovum, 

 but sometimes there are two. When mature, the ovum is ex- 

 pelled through the body- wall, and is fecundated by the sper- 

 matozoa, 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 Atliecata of 

 Hincks). The order Gorynida 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 deve- 

 lops a firm outer layer or l polypary.' No ' hydrothecce ' are 



