60 INVERTEBRATE ANIMALS. 



addition to this merely mecbiinical action, the tentacles appear to 

 exercise a bennmljing or even fatal influence upon the animals 

 grasped by them — this being apjiarently due to the thread-cells 

 with which they are furnished. The mouth in the Hydra opens 

 directly into a capacious cylindrical cavity, which is excavated along 

 the whole length of the body, and which is both the body-cavity and 

 the stomach in one. This cavity (fig. 28) is filled with water derived 

 from the exterior, and also with the nutritive particles derived from 

 the fooil. Indigestible fragments are rejected by the mouth, a sep 

 arate anal opening being wanting. A striking proof of the essen- 

 tially low position of the Jfijdra in the animal scale is to be found in 

 its extraordinary capacity of resisting mutilation, or, in fact, me- 

 chanical injury of any kind short of absolute annihilation. The 

 briefest illustration of this fact is all that can here be given, but 

 with that the name of Trembley of Geneva mu.st be associated. 

 This well-known observer, in a long series of experiments, most of 

 which have been successfully repeated by subsequent naturalists, 

 discovered that the Hydra could be mechanically divided with a 

 knife into any number of fragments, with the sole result that each 

 and all of these possessed the power of developing themselves into 

 fresh and independent polypites. Further, the animal could even 

 be turned inside out, with a necessary transposition of the ectoderm 

 and eniloderin, without any apparent inconvenience or interference 

 with its health. 



Reproiluetioii in the Jlijdra is efTeeted non-sexiially by gemmation, and 

 sexually Ij}' tlie jiroilaction of ova and sperm-cells ; the former process being 

 followed in summer and the latter in antunin, 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 or proximal extremity. These 

 bu'ls at first consist simply of a tubular prolongation of the ectoderm and 

 endoderni, enclosing a cavity which communicates with the general cavity of 

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

 this bud, and after a longer or shorter period the new Hydra, thus produced, 

 is detached to lead an independent lij'e. Each Hydra can produce many 

 such buds during the summer season, anil the liljerated buds can also repeat 

 the same process, so that in this way rei)roduction is rapidly carried on. In 

 the second or sexual metliod of reproduction, ova and sperm-cells are produced 

 towards the winter in external processes of the body-wall. The spermatozoa 

 are developed in little conical elevations (Hg. 30, 6), which are produced ne.ar 

 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 lixed or proximal ex- 

 tremity of the animal (Hg, 30, c). When mature, the ovum is fertilised by the 

 sperm-cells, both being set i'mQ into the water liy the rupture of the body-wall. 

 The embryo Hydra is at first covered with vibrating cilia, and swims freely 

 ulioni, until it meets with a suitable locality. It then fixes itself by one ex- 

 ti'ciiiity, llie cilia drn]) o(T, and a mouth and tentacles are developed at the 

 distal cud of (he body. 



