AND DEVELOPMENT OE MYRIOTHELA. 
555 
the caudal prolongations of the claviform tissue. These prolongations run to the surface 
of the muscular lamella, and become there intimately united with it, so that it is per- 
haps impossible to detach them without laceration ; but I cannot affirm any thing further 
regarding the nature of this union. But though Myriothela does not seem to afford any 
evidence of the direct continuation of the muscular fibrillae with the caudal prolonga- 
tions of the claviform tissue, it cannot be regarded as in any way contradicting the 
hypothesis that this tissue is destined for the reception of external stimulus — in other 
words, that it represents a nervous system. 
The general structure of the ectoderm of the Myriothela hydranth is that which has 
been now described ; in the globular capitula of the tentacles, however, we have a most 
singular modification of those structures which lie external to the hyaline lamella. 
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 
(fig. 3, a , & fig. 7), which are attached by their inner or apical ends to the hyaline 
lamella of the capitulum to which they are perpendicular, and thence radiating out- 
wards terminate at some distance from the outer boundary of the capitulum in a curved 
surface, which occupies somewhat more than a hemisphere. The distal or basal extre- 
mity of each pyramid is formed by a curve of greater convexity than that of the general 
surface formed by their combined bases ; and this surface thus acquires a minutely 
papillose appearance. The whole organ thus constituted caps the hyaline membrane 
and endoclerm of the summit of the tentacle. In its structure it strongly suggests the 
rod-like tissue which in higher animals we know to be associated with special organs of 
sense. 
Radiating from its convex surface are a multitude of slender filaments, which make 
their way among the cells of the ectoderm, and terminate distally at a short distance 
within the outer surface of the capitulum, where each carries on its summit an oviform, 
transparent, very thin membranous sac (fig. 3, b, h & fig. 8). This sac bears, close to 
its distal end, a minute bristle-like process, and is completely filled by a firm refringent 
capsule, within which may be seen a transparent cylindrical cord wound in two or three 
coils. The capsule (fig. 9) is easily liberated from its enveloping sac, and under slight 
pressure the contained cord may sometimes be ejected through its distal end (fig. 10). 
The whole assemblage of sacs, with their included capsules, forms a zone parallel to the 
surface of the capitulum and a little within it (fig. 3). 
The close resemblance of the capsule, with its contained cord, to a thread-cell is 
abundantly obvious ; and even the external sac, with its bristle-like process, has its 
parallel in the generating-cell of certain thread-cells. But besides the presence of the 
filiform peduncle there are other points in which these remarkable bodies differ from 
true thread-cells. The included cord does not, like the contents of an ordinary thread- 
cell, consist of a wider portion continuous with a narrower one, which during ejection 
becomes invaginated in the wider, but, on the contrary, possesses a uniform diameter 
considerably greater than that of the filament of an ordinary thread-cell ; and instead of 
