216 ANATOMY OF SPECIAL FORMS. 



This dilatation, at least in the young stage of the medusa, is more or less deeply plunged into the 

 substance of the umbrella. It is the " common cavity" of Huxley, the " chyme-receptacle" of 

 Gegenbaur, and the " atrium" of the present Monograph. 



If we suppose this atrium to become greatly developed and to extend itself laterally until it 

 occupies nearly the entire width of the umbrella summit or atrial region, while the concave por- 

 tion or manubrial region of the umbrella, as well as the free portion of the manubrium itself, attains 

 but slight development, we shall have as the result a medusa with the characteristic features of 

 that of ClavateUa} The umbrella will no longer present the free dilatable and contractile bell of 

 an ordinary medusa, while the radiating canals, which in the latter extend throughout nearly 

 the whole height of the umbrella, will in Clavalella, by the approximation of the atrium 

 to the circular canal, be reduced to the condition of the short tubular passages necessary to keep 

 up the communication between these two parts of om- singularly modified medusa. The marginal 

 tentacles agree with those of the natatory medusa in their position and relation to the gastro- 

 vascular system, but to compensate for the loss of locomotive function in the umbrella they take 

 this function upon themselves, and are fitted for the conveyance of the medusa from place to 

 place by acting as organs of reptation. 



The modification undergone by the manubrium will explain the situation of the ova, 

 apparently so very different from that of those bodies in the natatory medusa. In consequence of 

 the very imperfect development of a free pendulous manubrium, the generative elements which in 

 a natatory medusa would be developed within the cavity of the umbrella in the walls of the 

 manubrium are here, as it were, forced upwards and compelled to occupy a space immediately 

 beneath the ectoderm of the back, through which they at last escape by rupture ; though in the 

 species described by Claparede they approach more to the usual position, lying beneath the 

 ectoderm of the lower surface, and ultimately escaping at this part of the medusa. In all cases, 

 however, whether the medusa be natatory or ambulatory, the generative elements have their 

 origin between the ectoderm and endoderm. 



Cladonema radiatum. 

 Plate XVII. 



This singular and beautiful little hydroid was first described by Dujardin. It possesses 

 special interest not only as a well-marked type with striking peculiarities of structui-e, but as 

 having been the first in which the entire life-scries was followed through the complete succession 

 of hydranth, medusa, and planula, back to the hydranth again. 



The Hydropliyton. — The hydrophyton consists of very slender branched stems, which spring 

 from a creeping filiform stolon (figs. 1, 2). The whole is invested with a smooth perisarc. The 

 ramification, however, varies much, and specimens may frequently be seen consisting of a delicate 

 tubular filament adherent to the walls of the aquarium, over which it may extend for several 

 inches, while it sends off from distance to distance short simple hydranth-bearing ramuli. In 

 some of the specimens examined long slender branches, which showed no hydranths on their 



' Tlie comparison of the digestive cavity of the Ctoi'a/e//a-medusa with the " chyme-receptacle" 

 of Gegenbaur Las already been made by Krobn. 



