XV111 
THE VOYAGE OF H.M.S. CHALLENGER. 
them under the name of ganglion cells, and regards them as representing a differentiated 
nervous system. As seen in the tentacles of Eudendrium ramosum, they appear as well- 
defined nucleated masses of protoplasm, which send off outrunning tapering processes, 
usually three in number, and which under treatment with osmic acid appear thickly 
filled with black granules. Many of the outrunners, which the ganglion cells send off in 
the tentacles, unite with one another so as to form a plexus, while others lose them- 
selves between the muscle fibrillse, but no indubitable connection between nerve fibre 
and muscles has been found. Others again run to the thread-cells, and probably end 
close upon the cnidocil. 
In other parts of the hydranth the form of these cells is less definite, and here they 
usually become accumulated in small heaps which lie betw T een the ectoderm and meso- 
sarc, and from which at most a single process is sent off to run between the ectoderm 
cells. Jickeli has sometimes seen a filament running between the ectoderm cells to a 
very minute cell with dark granular protoplasm and small elongated nucleus. He 
regards this as a “sense cell.” Besides this mode of peripheral termination he believes 
that he has also seen one in the form of free nerve endings. 
The ganglion cells also occur in great profusion in the coenosare. Nothing, however, 
has anywhere been seen in any Hydroid trophosome which could with any probability be 
regarded as a specially differentiated nervous centre. 
Gland-Cells.— hi some places certain cells of the ectoderm appear to act as gland-cells. 
Such cells have been described by Weismann in some species of Eudendrium, where they 
form a complete ring round the base of the hydranth. They consist of a firm protoplasm 
which readily takes up colouring matter. 
In Tubularia larynx and some other species of Tubularia a bowl-shaped accumu- 
lation of cells, which forms a projecting collar, crowns the stem immediately below the 
hydranth like the capital of a pillar. 1 ; The cells composing this collar may also be regarded 
as gland-cells. A glandular function may also be attributed to peculiar elongated 
cells with a radial disposition which form the aboral extremity of the Actinula or free 
larva of Tubularia and of Myriotliela. There can be little doubt that these cells are 
destined to give origin to an excretion by which the Actinula becomes fixed at the close 
of its free locomotive existence. Cells of a similar nature would seem to be present in 
the aboral extremity of Hydra. 
Sarcostyles. — In connection with the ectoderm, perhaps more appropriately than 
elsewhere, may be described certain very remarkable zooids which are found throughout 
the whole of the Plumularinse, where they occur in the form of minute fleshy outgrowths 
which are contained in cup-shaped or tubular appendages of the stem, and are in direct 
communication with the coenosare. 
To these bodies special attention was called by Busk, who described them under the 
1 Gymnoblastic Hydroids, p. 407, pi. xxi. fig. 5. 
