REPORT ON THE HYDROIDA. xxiii 
spiral, while it is entirely destitute of the barbs or spines which constitute so character- 
istic a feature in the typical thread-cell. 1 
It is scarcely possible to avoid suspecting that these capsules with their associated 
structures, differing as they do from ordinary thread-cells, are destined for the perform- 
ance of some sensory function. 
The claviform tissue which is developed in the deeper parts of the ectoderm, and which 
is as we have seen in intimate relation with the muscular layer, is obviously comparable 
with the caudate cells of Kleinenberg’s neuro-muscle layer, and its deep position in the 
ectoderm will scarcely detract from the weight of this comparison when we bear in 
mind that the necessary stimulus may be carried to it through the cells which form the 
thin layer of ectoderm with which it is covered. There can be little doubt that the 
rod-like tissue into which the stalks of the capsules may be traced is a special modifica- 
tion of this claviform tissue, while it forcibly recalls the nerve-rods in the sense organs of 
higher animals. Though all the necessary connections of the parts have not yet been 
demonstrated, we see enough to make us believe that it is at least probable that 
the various structures here associated represent an apparatus for the reception and 
transmission of impressions received from without. 
Another very exceptional condition is seen in the presence of a well-marked layer of 
circularly disposed muscular fibrillse, w 3 * * * 7 hich are developed in the ectodermal coat which 
lies immediately on the generative mass in the gonophore of Myriothela, and by con- 
traction of which the contents of the gonophore on attaining maturity are expelled. 
3. The Mesosarc. 
Attention was first called to the existence of this layer in the Hydroid body by 
Reichert, who described it under the name of £: Stiitzlamelle.” 2 It is in its normal 
condition a perfectly hyaline structureless membrane which everywhere intervenes 
between the endoderm and the ectoderm, entering the tentacles along with these 
layers, and forming a cul-de-sac in the summit of each. It not only separates the 
peculiar chorda-like endodermal tissue of the tentacular axis from the surrounding 
ectoderm, but sends off at the base of the tentacle a layer which separates the chorda- 
like tissue from the proper digestive endoderm. The longitudinal muscular fibrillse 
of the ectoderm lie in close apposition with its outer surface. Its thickness may in 
some places be seen to be traversed by very delicate fibrils, which run vertically to its 
surfaces and form a connection between the two body layers, ectoderm and endoderm, 
which it separates from one another. 
In Monocaulus imperator, the gigantic Tubularian brought up by the Challenger 
1 Jickeli has described in Hydra certain small thread-cells whose filament on ejection remains coiled in a way very 
similar to that here seen. 
2 Carl B. Reichert, Uber die contractile Substanz und ihre Bewegungs-Erscheinungen, Berlin, 1867. 
