MANUAL OF ZOOLOGY. 



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 



Fig. 18. Morphology of Hydrozoa. a Diagrammatic section of Hydra. The dark 

 line is the ectoderm, the fine line and clear space adjacent are the endodeim. 

 b Hydra viridis, showing a single ovum contained in the body-wall near the 

 proximal extremity, and two elevations containing spermatozoa near the bases of 

 the tentacles ; c Hydra vulgaris, >with an undetached bud ; d Thread-cell of the 

 Hydra, greatly magnified. 



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 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 Hydra 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. 1 8, 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 



