84 



MANUAL OF ZOOLOGY. 



hension, being capable of retraction till they appear as nothing 

 more than so many warts or tubercles, and of being extended 

 to a length which is in some species many times longer than 

 the body itself. (In the Hydra fusca the tentacles can be 

 protruded to a length of more than eight inches.) Each con- 



Fi^. 12 — Morphology of Hydrozoa. a Diagrammatic section of Hydra. The 'dark 

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

 h H^dra 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 vulgarist with an undetached bud ; d Thread-cell of the 

 HydrUf greatly magnified. 



sists of a prolongation of both ectoderm and endoderm, en- 

 closing a diverticulum of the somatic cavity, and they are 

 abundantly furnished with thread-cells. The cylindrical hydro- 

 soma is excavated into a single large cavity, lined by the en- 

 doderm, and communicating with the exterior by the mouth. 

 This — the " somatic cavity " — is the sole digestive cavity with 

 which the Hydra is provided, the indigestible 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 gt-adually into 

 a new and perfect polypi te. 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 



