324 



HYDROID^. Part IV. 



its extreme frailty, it is often carried away by the decomposing hydra. In a fresh 

 state it is so closely applied to the wall of the chymiferous cavity, that it has 

 not been recognized. 



The reproductive calycles so closely resemble those of Obelia coramissuralis 

 (PI. XXXIV. Fie/. 11), described in the preceding section, that one figure may serve 

 for both. They arise singly from the base of the pedicels of the hydra; not 

 in the fork, between the latter and the main stem, but immediately above the 

 great thickening of the stem joint, and, alternating on successive joints, they 

 project at right angles to the plane in which the hydra-pedicels trend ; so that 

 , in a view from above we would have a cross, formed by the alternating hydra- 

 pedicels on one hand, and at right angles to this the two limbs formed by the ^ 

 alternating reproductive calycles. The pedicels of the latter are very short, and 

 consist of three or four rings; and the angle between them and the joints from 

 which they arise is not so great as that of the hydra-pedicels. Sometimes we 

 have found a reproductive calycle on each side of one and the same pedicel, and 

 this may be repeated four or five times on the same stem ; but, in such cases, there 

 was a strong tendency to branching, or throwing out stolon-like processes, or the 

 stem was actually branching. This Hydroid does not usually branch, but when it 

 occurs, the trend of the hydra-pedicels and of the reproductive calycles is at right 

 angles to that of the main stem, so that the hydra-pedicels of the main stem 

 trend in the same plane as the reproductive calycles of the branch, while the 

 reproductive calycles trend in a plane which, although at right angles to that of 

 the pedicels, cuts that of the branch-pedicels at a sharp angle, equivalent to the 

 angle between the stem and the branch. 



In every part of the hydrarium, the corneous sheath is composed of fibres, 

 arranged longitudinally (PI. XXXIV. Fi(j. 1), not only visible in profile («, a\ a% 

 but also in a face vieAV {b); moreover distinct fibres (e) may be torn off". Trans- 

 verse to these fibres, and most conspicuous in the thickening of the joints of the 

 stem, are very irregular branching and anastomosing lines, which have the appear- 

 ance of cellular tis.sue, but in a view perpendicular to the surface {b), they are 

 simply transverse to the fibres. We have not been able to satisfy ourselves as 

 to the nature of these lines. They are most frequent in old stems. 



Owing to the peculiarity of the joints of the stem, the common chymiferous 

 channel of the hydrarium alternately approaches the thinner side of each successive' 

 internode with a slightly sinuous course, and alternately diverges, immediately 

 above the great thickening of each joint, into the pedicels, and terminates in 

 a hydra, or, at the base of each pedicel, passes into the reproductive calycles, 

 where, in time, it produces medusaj-buds from its double walls. 



The hydrte {Figs. 3 and 4) have thirty-five tentacles, and, as in other Campanu- 



