Chap. II. ADULT HYDRA OF CLAVA. 219 



nocnidia, Hybocodon, Parypha, Corymorpha, and Rhizogeton, are not frequented by 

 Clava. It alone, among our Hydroids, though much less protected by a natural 

 covering than several of the Tubularians, is subjected to the dashing of the 

 breaking surf A little force suffices to lift the whole colony from its foundation, 

 to which it clings by the simple adhesion of its horny, stolon-like basis {Fig. 2, e). 

 The creeping stems are usually so closely interwoven, and agglutinated to each 

 other, by their horny sheaths, that, owing to the density of the mass, they cannot 

 be easily distinguished as tubular bodies; but upon the outskirts of the group, 

 where they are youngest, each one may be traced separately {Fig. 2, e). They 

 appear, to the naked eye, as having about the diameter of a common horsehair. 

 By actual measurement, they have an average diameter of y-|-5 of an inch. At 

 intervals of from ^^ to -^^ of an inch, the bases {d) of the upright portions of 

 the stems arise from the stolons, without expansion. About -^^ of an inch up the 

 stem it increases in diameter, rather suddenly, to about three times that of its 

 base, and, with this increase, it rises half an inch, in full-grown specimens, with 

 a very slightly tapering outline {Fig. 2, A B). 



The tip of each stem is terminated by an elongate oval head {Fig. 2, A c), 

 which is scarcely greater in diameter than the region below, and is provided with 

 long, round, slender, tapering, pointed tentacles, which are arranged in a close spiral, 

 and are often not less than thirty-five in number. During the breeding season, 

 the region just below the head is loaded with compound raceme-like bunches of 

 medusoids {b), which sometimes occupy one third of the length of the stem (wood- 

 cut 32, p. 221), but more commonly are crowded at the upper part. The younger 

 stems, up to an age when they have as many as twenty tentacles {Fig. 2, C E), 

 do not bear medusoids. Of all the Tubularians, this genus has the most slender, 

 and, in proportion to the size of the head, by far the longest tentacles. From 

 the mouth {Fig. D, c) at the tip of the head, to the attachment of the slender 

 base {d) of the stem, the whole upright body is highly contractile, and capable 

 of assuming a variety of shapes. When very lively it is stretched to the utmost 

 {Fig. 2, A), with elongated head (c), and extremely attenuated tentacles {a); at 

 other times, every thing remaining as in the first instance, the head is depressed 

 to a flatrtopped disk (B, c), from which the tentacles («) radiate nearly in one 

 plane, like the spokes of a wheel; or the stem is contracted to one half its 

 greatest height (H), and the tentacles of the flattened disk are reduced to one 

 fourth or fifth of their greatest elongation ; sometimes the region of the mouth 

 (D, c) is much extended, whilst the head, tentacles, and stem are reduced by one 

 half or more, and then in a moment the lips of the mouth {Fig. 8, g^) are rolled 

 outwards and backwards. When disturbed, the whole body assumes the most 

 contracted condition {Fig. 2, F) ; the stem and head shorten, and the tentacles 



