SPONGES AND ZOOPHYTES. 125 



obscured by a turbid film, which gradually thickens, so that at last its 

 form is wholly lost. The soft parts are soon completely dissolved, and 

 the harder indigestible portions are rejected through the mouth. A 

 second orifice has been observed at the lower extremity of the stomach; but 

 this would not seem to be 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 Hydrozoa, brings into mutual connection the 

 lower extremities of the stomachs of all the individual polypes (Plate xx.). 

 515. The ordinary mode of reproduction in this animal is by a ' gem- 

 mation ' resembling that of Plants. Little bud-like processes (Fig. 354, 

 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 ob- 

 served on the young polype before quitting its parent; and as many as 

 nineteen young Hydrcs in different stages of development have been seen 

 connected with a single original stock (Fig. 355). This process takes 

 place most rapidly under the influence of warmth and abundant food; it 

 is usually suspended 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 ex- 

 traordinary power which one portion possesses of reproducing the rest. 

 Into whatever number of parts a Hydra 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 ap- 

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

 ' germ-cell/ imbedded in a store of nutriment adapted for its early devel- 

 opment) 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 appear that sometimes one individual Hydra de- 

 velops only the male cysts or sperm-cells, while another develops only 

 the female cysts or ovisacs; but the general rule seems to be that the same 

 individual forms both organs. The fertilization of the ova, however, 

 cannot take-place until after the rupture of the spermatic cyst and of the 

 ovisac, by which the contents of both are set entirely free from the body 

 of the parent. The autumn is the chief time for the development of the 

 sexual organs; but they also present themselves in the earlier part of the 

 year, chiefly between April and July. According to Ecker, the eggs of 

 H. viridis produced early in the season, run their course in the summer 

 of the same year; while those produced in the autumn, pass the winter 

 without change. When the ovum is nearly ripe for fecundation, the 

 ovary bursts its ectodermal covering, and remains attached by a kind of 

 pedicle. It seems to be at this stage that the act of fecundation occurs; 

 a very strong elastic shell or capsule then forms round the ovum, the sur- 

 face of which is in some cases studded with spine-like points, in others 

 tuberculated, the divisions between the tubercles being polygonal. The 

 ovum finally drops from its pedicle, and attaches itself by means of a 

 mucous secretion, till the hatching of the young Hydra, which comes 



