228 



HYDROIDyE. 



Part IV. 



Fiij. 33. 



is so equally disposed, for, upon plunging the focus to the base of the upright indi- 

 viduals, a uniform layer of fleshy substance (PI. XVII. Figs. 1, K, 5, a, 5", c, 5°, a, 

 6, d) is found to occupy the whole length and breadth of the group. It is neither 

 in this layer, nor in its upward continuation, the outer wall of the individuals, that 

 the rosy tint lies, but in the interior of the thick-walled, closely anastomosing channels 

 {Figs. 5, b, 5% e, b% c, and 6, b, and PI. XXVI. Fig. 18, i A'). Unlike the hydroids 

 of other genera, those of Hydractinia are composed of no less than four different 

 forms of individuals. Premising, what has been ascertained for some time, that the 

 sexes are separate, all the individuals of one colony being either male or female, it 

 may be said that each colony is triraorphous. Taking a female colony (PI. XVI. 

 Fig. 1) for example, we find, first, the rej^roductive form (ABC F), with a globular 

 head (/<), of short spherical tentacles, along whose stem the egg-bearing medusaj (e) 



bud ; secondly, a form which is nothing more 

 than an extremely elongated reproductive 

 hydroid (E, and wood-cut 3o), with much 

 smaller heads than the generality of the first 

 form and a stem which is frequently branched 

 [e g). This form is only to be found on 

 the outskirts of the colony. However, be- 

 tween this form and the reproductive one 

 there are gradations, showing, as will be 

 pointed out hereafter, that, after all, this form 

 is hardly to be separated from the first. 

 Lastly comes the sterile form (D G H I), 

 with long, tapering tentacles, arranged in one 

 row, and a short proboscis {p). The fourth 

 form is found among the males (PL XVI. 

 Fig. 2); it is the sterile hydroid (D E F 

 G H I), with a long proboscis {p). Other- 

 wise the males and females resemble each 

 other. The degree of intermixture of the 

 fertile and sterile individuals vai'ies consid- 

 erably ; in some parts of a colony the}^ are 

 about equally distributed, whilst in others they 

 are either nearly all fertile, or nearly all 

 sterile. In all cases the hydroids are densely 

 packed together. 

 Underlying the whole colony is a laj^er of horny substance, either in the form of 

 a network, or of a uniform layer, with ridges upon the u^jper surfiice, anastomosing 



The outskirts of a hydromedusarium of Hydractinia 

 polyclina, to show the extremely elongated fertile hydrie 

 which fringe the border. Magnified 2.5 diameters. From 

 nature, by H. J. Clark. 



a the edge of the shell (Xatica) to which the colony is attached. 

 — 6 parallel ridges of the horn-like network. — c edge of the 

 horn-like layer. — dfh individual hydrfe coiled into one, two, 

 or three spiral turns. — t g two hydra.^ which are forked, and 

 have two head.-*. 



