60 INVERTEBRATE ANIMALS. 



Reproduction in the Hydra is effected non-sexually by 

 gemmation, and sexually by the production of ova and sperm- 

 cells ; the former process being followed in summer and the 

 latter in autumn, few individuals appearing to survive the 

 winter. In the first or non-sexual method, the Hydra throws 

 out one or more buds, usually from near the fixed extremity 

 (Fig. 13, c). These buds at first consist simply of a tubular 

 prolongation of the ectoderm and endoderm, enclosing a 

 cavity which communicates with the general cavity of the 

 body. A new mouth and tentacles are soon developed at the 

 free end of this bud, and after a longer or shorter period the 

 new If ydr a, thus produced, is detached to lead an independent 

 life. Each Hydra can produce many such buds during the 

 summer season, and the liberated buds can also repeat the 

 same process, so that in this way reproduction is rapidly 

 carried on. In the second or sexual method of reproduction, 

 ova and sperm-cells are produced toward the winter in ex- 

 ternal processes of the body-wall. The spermatozoa are de- 

 veloped in little conical elevations, which are produced near 

 the bases of the tentacles ; and the ova are formed in much 

 larger elevations, of which there is ordinarily but one, placed 

 nearer to the fixed extremity of the animal (Fig. 13, b). When 

 mature, the ovum is fertilized by the sperm-cells, both being 

 set free into the water by the rupture of the body-wall. The 

 embryo Hydra is at first covered with vibrating cilia, and 

 swims freely about, until it meets with a suitable locality. 

 It then fixes itself by one extremity, the cilia drop off, and 

 a mouth and tentacles are developed at the free end of the 

 body. 



OKDER II. CORYNIDA. In the second order of the Hydroid 

 zoophytes, known as the Corynida or Tubularida, \ve have a 

 number of organisms which in their essential structure are 

 closely related to the Hydra, but which differ considerably 

 in the nature of the reproductive process. All of them are 

 marine, with the single exception of the genus Cordylophora, 

 which inhabits fresh water. Some of the members of the 

 order are simple, consisting of no more than a single polypite. 

 In these cases there is an exceedingly close approach to the 

 structure of the common Hydra, but the polypite is per- 

 manently fixed without the power of voluntarily changing its 

 place, while the reproductive process is considerably different. 

 In the majority of the Corynida, however, the hydrosoma is 

 compound, consisting of a greater or less number of separate 



