HYDROZOA 867 



sacs ; but the general rule seems to be that the same individual 

 forms both organs. The fertilisation 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 free. 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 be- 

 tween April and July. According to Eeker, 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 t^ be at this stage that the act of 

 fecundation occurs ; a very strong elastic shell or capsule then 

 forms round the ovum, the surface 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 forth provided with four 

 rudimentary tentacles like buds. The Hydra possesses the power of 

 free locomotion, being able to remove from the spot to which it has 

 attached itself to any other that may be more suitable to its wants ; 

 its changes of place, however, seem rather to be performed under the 

 influence of light, towards which the Hydra seeks to move itself, than 

 with reference to the search after food. 1 



The compound Hydroids may be likened to a Hydra whose 

 gemmae, instead of becoming detached, remain permanently connected 

 with the parent ; and as these in their turn may develop gemma? 

 from their own bodies, a structure of more or less arborescent 

 character, termed a poly par y, may be produced. The form which 

 this will present, and the relation of the component polypes to each 

 other, will depend upon the mode in which the gemmation takes 

 place ; in all instances, however, the entire cluster is produced by 

 continuous growth from a single individual ; and the stomachs of the 

 several polypes are united by tubes, which proceed from the base of 

 each, along the stalk and branches, to communicate with the cavity 

 of the central stem. Whatever may be the form taken by the stem 

 and branches constituting the polypary of a hydroid colony, they will 

 be found to be, or to contain, fleshy tubes having two distinct layers, 

 the inner (endoderm) having nutritive functions ; the outer (ecto- 

 derm) usually secreting a hard cortical layer, and thus giving rise 

 to fabrics of various forms. Between these a muscular coat is some- 

 times noticed. The fleshy tube, whether single or compound, is called 

 a ceenosarc, and through it the nutrient matter circulates. The 

 * zboids,' or individual members of the colony, are of two kinds : one 

 the polypite, or alimentary zooid, resembling the Hydra in essential 



1 A very full account of the structure and development of Hydra has been 

 published by Kleinenberg, of whose admirable monograph a summary is given by 

 Professor Allman, with valuable remarks of his own, in Qua rt. Journ. Microsc. Sci. n.s. 

 vol. xiv. 1874, p. 1. See also the important paper by the late Mr. Jeffery Parker 

 already cited. On the chlorophyll corpuscles of H. viridis consult Brandt, Mittli. 

 Zool. Stot. Neapel, iv. p. 191 ; Hamann, Zool. Anzeig. vi. p. 367; and Lankester 

 Quart. Journ. Microsc. Sci. n.s. xxii. p. 229. 



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