HISTOLOGY OF THE ENDODERM. 127 



fill it; while at a little distance from the base the encroachment of the cndodcrmal cells is usually 

 not so complete, and the cavity of the tentacle, though greatly contracted, still continues 

 j)ervious, and admits into it fluid with suspended particles from the digestive cavity of the 

 hydranth. 



Many of the cells which thus constitute the peculiarly modiiicd endodcrin of the tentacle 

 contain a very distinct uucleolated nucleus, roimd which is generally collected a colourless finely 

 granular protoplasm, which is often kept in connection with the cell-wall by radiating pro- 

 longations. A few large coloured granules are also usually contained in the cells. Towards the 

 distal extremity of the tentacle, where the septa are most regular and complete, we generally find a 

 little granular mass collected in the centre of the distal surface of each septum, or else forming a 

 little axile column stretching from septum to septum. These septa, with their intervening 

 chambers, have much the appearance of large cells which stretch entirely across the tube of the 

 tentacle ; it is possible, however, that they may be formed by the coalescence of two or more cells 

 from opposite sides, as in the more proximal portions, but here accompanied by the disappearance 

 of their walls in the axis of the tentacle, and the consequent confluence of their cavities. 



In some other cases the endodermal cells are not developed to such an extent as to obliterate 

 in any portion the cavity of the tentacle, which thus remains pervious in its entire length ; but the 

 large size and regular form of these cells continue to confer on the tentacle a distinctly septate 

 appearance. I have found this condition in the tentacles of Garveia nutans (PI. XII, fig. (5). 



In the planoblast also the tentacles usually present the same septate appearance as in the 

 hydranth, the endoderm being frequently so developed as entirely to obliterate the tube. Examples 

 of this may be seen in the marginal tentacles of most medusae, as, for instance, those which 

 constitute the planoblasts of Obdla dichofoma, and of Podocoryne carnea, as well as in the ten- 

 tacles of the free sporosac of Dicoryne. Those cases, however, in which the tentacle pre- 

 serves a complete continuity of its tube are much more common in the medusa than in the 

 hydranth. In the medusa, indeed, it is far from uncommon ; we find it, for example, in the 

 marginal tentacles of the medusa of Syncoryne eximia (PI. V, fig. 4), Gemmaria implcxa (PI. VII, 

 fig. 4), and Sarsia stranyula/a (woodcut, fig. 17), and, according to Haeckel, in the long extensile 

 tentacles of the Gcryonidce} 



In the bulb-like expansion which usually exists at the base of the marginal tentacles of the 

 medusae, at the point where the radiating canal meets the circular canal, the endoderm acquires 

 increased thickness, and often forms lobulated projections which encroach upon the cavity, but never 

 obliterate it (woodcut, fig. 56). These projections are usually rich in cells filled with coloured 

 granules, which, like the coloured granules of the digestive cavity, appear to be secreted in 

 secondary cells developed in the interior of mother-cells. 



The endoderm lining the remainder of the gastrovascular canals is, on the other hand, entu-ely 

 destitute of lobes or ridges. It is very thin, and would seem to have a purely distributive 

 function, the fluid contents of the canals being propelled by vibratile cilia which clothe their 

 walls. These cilia may be well seen in Syncoryne eximia (woodcut, fig. 50). 



' Eeichert (loc. cit.) maintains the entire absence of the endoderm in the tentacles of the sertu- 

 larian and campanularian hydranths, ivhile he regards the septate appearance as produced by simple 

 extensions of his " supporting lamella," which, according to him, forms a layer secreted on the inner 

 surface of the tentacular ectoderm. This view, however, is opposed by the structure, which easily 

 admits of demonstration in the tentacles of many tubulariau hydroids. 



