. Contributions to the natural history of the United States of America. Zoology; Chelonia (Genus); Ctenophora; Cnidaria; Animals. 228 HYDROID^. Part IV. Fiij. 33. is so equally disposed, for, iijjon 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 chann
. Contributions to the natural history of the United States of America. Zoology; Chelonia (Genus); Ctenophora; Cnidaria; Animals. 228 HYDROID^. Part IV. Fiij. 33. is so equally disposed, for, iijjon 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, h, b\ e, 5", e, and 6, h, and PI. XXVI. Fig. 18, b P). Unlike the hydroids of other genei'a, those of Hydi-actinia 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 trimorphous. Taking a female colony (PI. XVI. Fig. 1) for example, Ave find, first, the reproductive form (A B C F), with a globular head {/i), of short spherical tentacles, along whose stem the egg-bearing meduste (c) bud; secondly, a foim which is nothing more than an elongated reprodvxctive hydroid (E, and wood-cut 33), 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 colon}'. 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 (/j). The foiu'th form is found among the males (PI. XVI. Fig. 2); it is the sterile hydroid (D E F G H I), with a long jjroboscis (/>). Other- wise the males and females resemble each other. The degree of
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