210 ANATOMY OF SPECIAL FORMS. 



consideralile interval in front of this zone, and a little below the mouth, is a bnish-Iike group of 

 about eighty very contractile tentacles, much smaller than the proximal ones, and arranged in six 

 or seven closely placed alternate verticils. 



When a longitudinal section (fig. 7) is made through the hydranth from the mouth to the 

 stem, it will be seen that the body, as far back as the zone of proximal tentacles, presents a con- 

 tinuous cavity (a) lined by small spherical cells containing coloured granules. The floor of this 

 cavity projects into it in the form of a broad conical elevation. This conical projection consists 

 beneath its covering of small pigment-bearing cells, of a large-celled and clear-celled endoderm (e), 

 and the same tissue is continued backwards as far as the summit of the stem. A careful examina- 

 tion will show that this cellular mass, which frequently seems to fill up the proximal part of the 

 hydranth, is perforated in its axis by a tubular prolongation of the cavity of the distal part, 

 though, in consequence of temporary obliteration caused by the approximation of its walls, this 

 axial tube is usually difficult to detect. Like the cavity of the distal portion of the hydranth, it 

 is lined with small spherical cells {/) filled with coloured granules. It is continued towards 

 the summit of the stem, and then, becoming wider (3), receives the longitudinal canals (e), which 

 have been already described as traversing the stem in its entire length. 



The tentacles forming the proximal zone are destitute of any trace of a cavity (fig. 8), and 

 consist of a simple extension, with little change of form, of the large-celled colourless endoderm of 

 the body, surrounded by a layer of ectoderm. The distal tentacles seem to admit the cavity 

 of the hydranth for a short distance into their interior, but the tube soon becomes obliterated, its 

 axis at the same time assuming the usual septate appearance (fig. 9). 



The same difference of structure between the proximal and distal sets of tentacles exists 

 here as in Tahularia, and I regard the distal set in both cases as the true equivalents of the 

 tentacles in Eudendrium, Coryne, &c., while the proximal set must be viewed as superadded 

 structures. 



In fully developed specimens the hydranth measures about one inch across the proximal or 

 posterior coronal of tentacles. The colour of the brownish-red contents of the cells which line its 

 cavity is visible through the transparent ectoderm as far back as the proximal zone of tentacles. 

 Similarly coloured granules exist in the stalks of the gonophores, from which they may be traced 

 into the manubrium of the medusa, and, while this is still young, into its radiating canals. The 

 whole of that portion of the hydranth which lies behind the long tentacles is nearly colourless, 

 the pigment-containing cells being here obscured by the thickness of the colourless layer of 

 endoderm which lies external to them. 



The Gonophores. — The gonophores (figs. 1, 3, 4, 5, 5") are grouped in crowded clusters on the 

 extremities of branched tubular stalks which form their peduncles. The axis of these peduncles 

 is occupied by a continuous tube, which communicates freely with the cavity of the hydranth. 

 They are usually from fifteen to twenty in number, and are situated immediately within the zone 

 of proximal tentacles in two alternating series (fig. 1 and fig. 7, /)• 



I have not been able to demonstrate in the gonophores any trace of an ectotheca, and must 

 therefore regard them as truly naked. They are planoblasts, and when arrived at that stage in 

 which they are about to detach themselves from the stalk and become free (figs. 3, 4), they present 

 a deep umbrella, having its summit continued into a short conical projection traversed by a narrow 

 canal, which had kept the cavity of the manubrium in connection with that of the stalk. There 

 are four radiating canals, each of which expands into a bulb at the point where it enters the 



