of Hydroids from the White Sea. 221 



able quantity of thread-cells, and of a thin chitinous membrane, 

 covering this mass and expanding into the hjdranth and its 

 short stems (PI. V. fig. 2, d). The hydrorhiza is of a dark 

 greyish colom-, and is not covered with spines as in Hydrac- 

 tinia. The hydranths proceed from the hydrorhiza ; and 

 between them are placed the gonophores (PI. V. figs. I, 2). 



The principal interest of this hydroid is concentrated in the 

 hydranth, which is of an almost cylindrical shape, rather 

 thinner towards the lower, and abruptly truncated at the 

 upper part. It is attached to the hydrorhiza without the aid of 

 any well-defined hydrocaulus, unless we consider as such the 

 thick chitinous membrane covering the inferior part of the 

 hydranth (PI. V. fig. 2, d)^ and which, in fact, is but the con- 

 tinuation of the perisarc. The length of a full-grown speci- 

 men is 2 millims. The mouth (PI. V. fig. 2, a), a simple 

 regular round aperture, opens at the summit of the body, its 

 diameter being equal to that of the body. I have been unable 

 to detect any trace of a joint, or, in fact, any thing sepa- 

 rating the body from the hypostome. The only tentacle 

 (PI. V. fig. 2, h) the hydranth is provided with is attaclied to the 

 upper third of the body, from which it springs at a small angle: 

 its base is several times the thickness of its extremity ; and it 

 grows gradually thinnerto about half its length. The extremity 

 is not bulbous. The length of the tentacle increases with age, 

 and in full-grown specimens attains two or three times the 

 length of the whole body. Such prodigiously long tentacles 

 had never been previously remarked in any species of hydroids, 

 and this phenomenon finds an explanation in the following 

 circumstances: — 1st, that the hydroid being provided Avith a 

 single tentacle, the whole nutritive force is concentrated in it, 

 instead of being distributed amongst several, as in other cases; 

 2nd, that the hydroid being compelled to secure aliments 

 and to defend itself against its natural foes, this organ assumes 

 naturally an abnormal development under the greater activity 

 it is called upon to exercise. 



As to the histological structure, PI. VI. fig. 8 gives a trans- 

 verse section of the hydranth, showing that it differs in no 

 way from the ordinary structure. We see the mass of the 

 ectoderm, with the nuclei of the cells imperfectly defined; in 

 the ectoderm a considerable number of thread-cells (shown in 

 fig. 5) are dispersed all over the tentacles and the body, parti- 

 cularly on the upper part and round the buccal orifice. This 

 layer of ectoderm is succeeded by a mass of mesoderm 

 forming the muscular substance of the animal ; and this is to be 

 remarked on the surface, on account of its being striated ; and, 

 lastly, we remark the endoderm formed by small circular cells, 



