So MANUAL OF ZOOLOGY. 



little distance below the margin of the oral aperture. The 

 tentacles vary in number from five to twelve or more, and 

 they vary considerably in length in different species, being 

 much shorter than the body in the Hydra viridis, but being 

 extremely long and filamentous in Hydra fusca. They are 

 highly extensile and contractile, and serve as organs of pre- 

 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- 

 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 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 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. 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 



