CLADONEMA RADIATUM. 217 



summits, and had the pcrisarc, a little beyond the origin of the branch, thrown for a short dis- 

 tance into spiral folds, were given off here and there from the hydrocaulus. I can scarcely, 

 however, regard these branches as normal ; and as all the cases in which the trophosome of 

 Cladonema radiatum has been observed were those of specimens which had shown themselves in 

 the confinement of the aquarium, it is not improbable that the growth of these adventitious 

 branches is the result of the imnntural conditions to which the hydroid had been there exposed. 



The Ilydranths. — The hydranth, which is somewhat fusiform in shajie, has its tentacles dis- 

 posed in two verticils, with four tentacles arranged crucially in each. The two verticils differ from 

 one another in the form of the tentacles. In the distal verticil, which surrounds tiie base of a well- 

 developed hypostome, the tentacles terminate in spherical capitula, crowded with thread-cells ; 

 while in the proximal verticil, which is situated close to the base of the hydranth, the tentacles 

 are much smaller than those of the distal verticil, are less contractile, and are destitute of 

 capitula. The endoderm of both sets is septiform, with some opaque granules imbedded in 

 protoplasm which occupies the axis. 



The Gonophores. — The gonophores (figs. 3 — 10) are phancrocodonic, and present us with an 

 interesting and remarkable type of hydroid medusas. They bud from the hydranth immediately 

 above the proximal verticil of tentacles. 



After detachment from the trophosome these curious planoblasts easily attain to maturity 

 in the aquarium, differing in this respect from most other planoblasts, which in the confinement 

 of the aquarium soon perish, without ever attaining their mature condition. 



The mature planoblast of Clado7irma (figs. 3, 4, 10) is about a hne and a half in height. 

 When in repose the vertical and transverse diameters of the umbrella are nearly equal to one 

 another, but during systole the vertical considerably exceeds the transverse. The walls are thin, 

 and the outer and inner surfaces nearly parallel. A small boss-like hemispherical elevation projects 

 from the outer surface of the summit. 



The sub-umbrella is of a pale raw-sienna colour, and under the microscope may be seen to be 

 marked with distinct stria?, which take a transverse, somewhat wavy course between every two 

 radiating canals. They appear to indicate the action on the sub-umbrella of the contractile tissue 

 of its walls. 



The velum is very wide, the diameter of its central aperture when most diminished being 

 scarcely more than one eighth of the entire diameter of the codonostome (fig. 8). It is marked 

 by very distinct concentric striae, intersected by radiating striae, which extend from the margin of its 

 central aperture to its outer edge. Both these sets of striae represent the contractile fibrillated 

 tissue of the velum. 



The manubrium manifests great contractility. It is sometimes drawn together, so as to 

 present the appearance of an irregular globular mass in the top of the bell, and at other times 

 extended so as to touch with its distal end the surface of the velum, or even become protruded 

 beyond it. When extended it is seen to consist of two regions, a proximal and a distal (figs. 3, 4j. 

 The proximal region occupies about two thirds of the entire length of the manubrium. It is 

 somewhat fusiform when viewed from the side, but in transverse section it is seen to be penta- 

 hedral. It is in this portion alone that the generative elements are developed, and their presence 

 gives rise to five hemispherical projections, by which the manubrium becomes encircled near 

 its middle. The distal portion constituting the remaining third is very much thinner, and is 

 nearly cylindrical, though tapering slightly towards the mouth. The oral margin is continued 



