868 SPONGES AND ZOOPHYTES 



structure, and more or less in aspect ; the other, the gonozooid, or 

 sexual zooid, developed at certain seasons only, in Ituds of particular 

 shape. 1 



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

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

 the latter being again divided into the Tkecaphora and the Gymno- 

 chroa. In the first, neither the ' polypites ' nor the sexual zooids 

 l>ear true protective 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 con- 

 tains the Hydras, there is no polypary, and the reproductive zooids 

 (gonozooids) are always fixed and developed in the body-walls. Ac- 

 cording to Mr. Hincks, the two sexes are sometimes borne on the 

 same colony, but more commonly the zoophyte 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 from it. The ova, when fertilised by the spermatozoa, 

 undergo ' segmentation ' according to the ordinary type, the whole 

 yolk-mass subdividing successively into two. four, eight, sixteen. 

 thirty-two or more parts, until a ' mulberry mass ' is formed; this 

 then begins to elongate itself, its surface being at first smooth and 

 .showing a transparent margin, but afterwards becoming clothed with 

 cilia, by whose agency these little plan-tiler, closely resembling ciliated 

 Infusoria, first move about within the capsule, and then swim forth 

 freely when liberated by the opening of its mouth. At this period 

 the embryo can be made out to consist of an outer and an inner 

 layer of cells, with a hollow interior ; after some little time the cilia 

 disappear, and one extremity becomes expanded into a kind of disc- 

 by which it attaches itself to some fixed object ; a mouth is formed, 

 and tentacles sprout forth around it ; and the body increases in length 

 and thickness, so as gradually to acquire the likeness of one of the 

 parent polypes, a t'ter which the ' polypary ' characteristic of the genus 

 is gradually evolved by the successive development of polype-buds 

 from the first-formed polype and its subsequent offsets. The Medusa- 

 of these polypes (fig. (363) belong to the division called 'naked-eyed.' 

 on account of the eye-spots usually seen surrounding the margin of 

 the bell at the l>asr of the tentacles. 



A characteristic example of this production of medusa-like 

 'gonozooids' is presented by the form termed SyncOryne X((rsii (fig. 



1 A useful li^ti nl the principal terms used in describing liydroids, u ith definitions, 

 will be t'liuud cm \>\>. l(i and 17 of Professor Allmun's lu/xni mi the Hi/droida (Pin- 

 i of Ilic CLu/li HI/IT. 



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