866 



SPONGES AND ZOOPHYTES 



of the stomach ; but this would not seem to he properly regarded as 

 anal, since it is not used for the discharge of such exuviae ; it is 

 probably rather to be considered as representing, in the Hydra, the 

 entrance to that ramifying cavity which, in the compound Hijdrozoa, 

 brings into mutual connection the lower extremities of the stomachs 

 of all the individual polypes. 



The ordinary mode of reproduction in this animal is by a ' gemma - 

 tion ' resembling that of plants. Little bud-like processes (fig. 658, 

 b, c) developed from its external surface gradually come to resemble 

 the parent in character, and to possess a digestive sac, mouth, and 

 tentacles ; for a long time, however, their cavity is connected with 

 that of the parent, but at last the communication is cut off by the 



closure of the canal of the foot- 

 stalk, and the young polype quits 

 its attachment and goes in quest 

 of its own maintenance. A 

 second generation of buds is 

 sometimes observed on the young 

 polype before quitting its parent ; 

 and as many as nineteen young 

 Hijdr(t' in different stages of 

 development have been seen thus 

 connected with a single original 

 stock (fig. (5(50). This process 

 takes place most rapidly under 

 the influence of warmth and 

 abundant food ; it is usually sus- 

 pended in winter, but may be 

 made to continue by keeping the 

 polypes in a warm situation and 

 well supplied with food. Another 

 very curious endowment seems to 

 depend on the same condition 

 the extraordinary power which 

 one portion possesses of repro- 

 ducing the rest. Into whatever 

 number of parts a Ili/dra may 

 be divided, each may retain its 

 vitality, and give origin to a 



new and entire fabric ; so that thirty or forty individuals may 

 be formed by the section of one. The Hydra also propagates itself, 

 however, by a truly sexual process, the fecundating apparatus, or 

 vesicle producing 'sperm-cells,' and the ovum (containing the ' germ- 

 cell,' imbedded in a store of nutriment adapted for its early develop- 

 ment), being both evolved in the substance of the walls of the 

 stomach -the male apparatus forming a conical projection just 

 beneath the arms, while the female ovary, or portion of the body- 

 substance in which the ovum is generated, has the form of a 

 knob protruding from the middle of its length. It would appeal- 

 that sometimes one individual Hydra develops only the male cysts 

 or sperm-cells, while another develops only the female cysts or ovi- 



mi 



FIG. 660. Hi/dni fused in gemmation: a, 

 mouth ; b, base ; c, origin of one of the 

 buds. 



