CCELENTERATA : HYDRIDA. 113 



filamentous in Hydra ftisca. They are highly extensile and 

 contractile, and serve as organs of prehension, 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 consists of a prolongation of 

 both ectoderm and endoderm, enclosing a diverticulum of the 

 somatic cavity, and they are abundantly furnished with thread- 

 cells. The cylindrical hydrosoma (fig. 37, B) is excavated into 

 a single large cavity, lined by the endoderm, and communi- 

 cating 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 (fig. 37, A) 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 independ- 

 ent 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. 38). 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 

 by the spermatozoa, which are simultaneously liberated. The 



H 



