AND DEVELOPMENT OE MYEIOTHELA. 
553 
2. Ectoderm. 
Under this head I shall include, not only the proper cellular ectoderm, but the 
hyaline lamella which forms its internal boundary, and is composed of a fibrillated or 
muscular stratum, with a supporting structureless membrane. 
The proper cellular ectoderm (Plate 56. fig. 1, c, & fig. 2, cj , li) forms a much thinner 
zone than the endoderm. It is composed of two distinct strata — a superficial and a deep. 
The superficial stratum (fig. 2, g) consists of small round cells, several in depth. These 
are destitute of membrane, and contain abundance of yellowish corpuscles ; while on 
the summit of the tentacles (fig. 3), and in irregular patches on other parts of the body, 
they contain dark brownish-purple pigment granules. 
Lying irregularly among these ectodermal cells, and chiefly towards the free surface 
of the ectoderm, are the thread-cells (figs. 2 & 3). Two forms of thread-cells may be 
distinguished, — one oviform (fig. 4, a, a'), with the invaginated sheath occupying the 
axis ; the other fusiform (fig. 4, b, V), with a slightly curved axis, and having the invagi- 
nated sheath oblique. Both kinds of thread-cells are formed in the interior of certain 
cells belonging to the superficial layer of the ectoderm, and may be seen, some lying 
free among the true cells of this layer, others enclosed in their generating-cells, and 
either completely immersed in the granular matter of the cell or surrounded by a large 
clear vacuole (fig. 5). No facts, however, have come to my knowledge tending to throw 
further light on the mode of origin of the thread-cells. 
The deep layer of the cellular ectoderm (fig. 2, h) is formed by a very remarkable tissue, 
to which I shall refer under the designation of the claviform tissue. This is composed 
of cells consisting of a yellowish granular protoplasm, entirely destitute of membrane, 
and each drawn out into a long caudal process. They are frequently provided with an 
obvious nucleus. By the union of their caudal processes groups of claviform cells (fig. 6, a) 
are produced whose common stalk runs to the hyaline lamella, where it loses itself in 
the fibrillated stratum ( b ). The whole forms a very soft, pulpy, and somewhat glandular- 
looking tissue, easily broken down under the compressor. 
Caudate cells, of apparently the same significance, were first made known by Klein- 
enberg*, who discovered them in Hydra, where he believes that he has followed their 
caudal prolongations into direct continuity with the fibrillee of the muscular lamella. 
He regards the body of the cell as destined for the reception of stimulus from without, 
and, looking upon the whole cell with its fibrilliform continuation as representing a 
combined nervous and muscular system, he gives it the name of “neuro-muscle-cell.” 
According to this view Hydra would represent in the phylogenesis of animals a form in 
which the nervous and muscular tissues are as yet but imperfectly differentiated from 
one another. 
I believe that we are quite justified, with our present data, in attributing to the clavi- 
form tissue the general function of a nervous system. Indeed I do not see what other 
place it is possible to assign to it in the economy of the animal. In Myriothela, however, 
* Hydra, eine anatomisek-entwickelungsgeschichtliche Untersuchung. Leipzig, 1872. 
