XXII 
THE VOYAGE OF H.M.S. CHALLENGER. 
It lias just been said that the caudal prolongations of the clavate tissue may be 
traced into the fibrillated layer, but I have not succeeded in satisfying myself that tbe 
fibrillse are directly continuous with these prolongations, as Kleinenberg has shown to be 
the case with the ectodermal fibrillse of Hydra, in their relations to the caudate ecto- 
dermal cells of this genus, which, notwithstanding their superficial position, admit of an 
obvious comparison with the clavate tissue of Myriothela. 
The general structure of the ectoderm of Myriothela is that now described. In the 
globular capitula, however, which terminate the tentacles, we have a most singular modi- 
fication of those structures which lie external to the hyaline mesosarc. Here the place 
of the caudate cells is taken by a remarkable tissue, composed of closely appressed 
transparent prisms, or, to speak more exactly, of greatly elongated pyramids, which are 
attached by their apical ends to the mesosarc of the capitulum, and thence radiating 
outwards, terminate at some distance within the outer boundary of the capitulum in a 
convex surface, which slightly exceeds that of a hemisphere in extent. The whole body 
thus formed by this columnar tissue caps the hyaline mesosarc and subjacent endoderm 
of the summit of the tentacle. 
Radiating from its convex surface, a multitude of cnidopods may be seen. These make 
their way among the cells of the ectoderm, and terminate distally at a short distance within 
the surface of the capitulum, where each carries on its summit a peculiarly modified large 
thread-cell with its enclosing cnidocyst. The cnidocyst carries close to its distal end a 
well-developed cnidocil, and is completely filled by the firm refringent capsule, within which 
may be seen a transparent cylindrical chord wound in two or three coils. The capsule is 
easily liberated from its enveloping sac, and under slight pressure the contained chord may 
sometimes be ejected through its distal end. The whole assemblage of sacs, with their in- 
cluded capsules, forms a zone parallel to the surface of the capitulum and a little within it. 
Notwithstanding the indubitable relationship of the bodies just described to ordinary 
thread-cells, they differ from these in some important points which would lead us to 
believe that some function has been assigned to them, which is not that of the defensive 
or offensive office of the thread-cell. 
The difference between them and the thread-cells, as usually seen in the Hydroida, 
is sufficiently obvious. The included chord does not, like the filament of an ordinary 
thread-cell, consist of a wider portion continuous with a narrower one, which during 
ejection becomes evaginated through the wider, but on the contrary possesses a uniform 
diameter considerably greater than that of the filaments of the typical thread-cell, and 
instead of presenting, as in the latter, a vast multitude of coils rolled together into a 
regular spiral or into a complicated mass it has only two or three such coils. Further, 
when ejected from the capsule — while it still holds on by one end to the point of exit — 
it does not, like the filament of an ordinary thread-cell, straighten itself, and shoot across 
the field of the microscope, but immediately on becoming free coils itself again into a 
