126 THE MICROSCOPE AND ITS REVELATIONS. 



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 suita- 

 ble to its wants; its changes of place, however, seem rather to be per- 

 formed under the influence of light, towards which the Hydra seeks to 

 move itself, than with reference to the search after food. 1 



516. 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 gemmae from 

 their own bodies, a structure of more or less arborescent character, termed 

 a polypary, 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 pro- 

 ceed 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 sometimes 

 noticed. The fleshy tube, whether single or compound, is called a cceno- 

 sarc; and through it the nutrien.t matter circulates. The ' zooids,' or 

 individual members of the colony, are of two kinds: one, the polypite, or 

 alimentary zooid, resembling the Hydra in essential structure, and more 

 or less in aspect; the other, gonozooid, or sexual zooid, developed at cer- 

 tain seasons only, in buds of particular shape. 



517. The simplest division of the Hydroida is that adopted by Mr. 

 Hincks, 2 who groups them under the sub-order Athecata and Thecata, 

 the latter being again divided into the Thecaphora and the Gymnochroa. 

 In the first, neither the 'polypites 5 nor the sexual zooids bear true pro- 

 tective cases; in the second, the polypites are lodged in cells, or, as Mr. 

 Hincks prefers to call them, calycles, many of which resemble exquisitely 

 formed crystal cups, variously ornamented, and sometimes furnished with 

 lids or opercula; in the third, which contains the Hydras, there is no 

 polypary, and the reproductive zooids (gonozooids) are always fixed and 

 developed in the body-walls. According to Mr. Hincks, the two sexes 

 are sometimes borne on the same colony, but more commonly the zoo- 

 phyte is dioecious. The cases, however, are much less rare than has been 

 supposed, in which both male and female are mingled on the same 

 shoots. The sexual zooids either remain attached, and discharge their 

 contents at maturity, or become free and enter upon an independent 

 existence. The free forms nearly always take the shape of Medusce (jelly 

 fish), swimming by rhythmical contractions of their bell or umbrella. 

 The digestive cavity is in the handle (manubrium) of the bell; and the 

 generative elements (sperm-cells or ova) are developed either between the 

 membranes of the manubrium, or in special sacs in the canals radiating 



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

 been published by Kleinenberg; of whose admirable Monograph a summary is 

 given by Prof . Allman, with valuable remarks of his own, in "Quart. Journ. 

 Microsc. Sci.," N.S., Vol. xiv. (1874), p. 1. See also the important Paper by Mr. 

 Jeffery Parker already cited. 



* " History of British Hydroid Zoophytes," 1868. 



