STRUCTURE OF HYDRA. 53 
which are prolongations of the body-wall, and are hollow, 
communicating with the body-cavity. 
Such is the general structure of the Hydra. In the 
ectoderm are situated the lasso-cells or nettling organs, be- 
ing minute barbed filaments coiled up in a cell-wall, which 
may be thrown out so as to paralyze the animals serving as 
food. While the endoderm forms a simple cell-layer, the 
outer layer (ectoderm) is more complex, as just within an 
external simple layer of large cells is a. multitude of smaller 
cells, some of them being thread or lasso-cells, while still 
within are fine muscular fibrille which form a continuous 
layer. The large cells first named end in fibre-like pro- 
cesses, which alone possess contractility, and are thought by 
Kleinenberg to be motor-nerve endings. But these cells, 
once termed “‘nerve-muscle cells,” do not combine the func- 
tions of muscle and nerve. ‘The little cavities between 
the large endodermal cells and the muscular layer (meso- 
derm?) which lies next to the endoderm are filled with 
small cells and lasso-cells, forming what Kleinenberg calls 
the interstitial tissue.‘ From this tissue are developed the 
eggs and sperm-cells. 
The body being but slightly differentiated or set apart 
into special organs, the Hydra, like other low creatures, is 
capable to a wonderful degree of reproducing itself when 
artificially dissected. Trembley, in 1744, described in his 
famous work how he not only cut Hydras in two, but on 
slicing them across into thin rings, found that from each 
ring grew out a crown of tentacles; he split them into lon- 
gitudinal strips, each portion becoming eventually a well- 
shaped Hydra, and finally he turned them inside out, and 
in a few days the evaginated Hydra swallowed pieces of 
meat, though its old stomach-lining had now become its 
skin. We shall see that not only many Hydroids, Aca- 
lephs, some Echinoderms, and many worms, may reproduce 
lost parts and suffer artificial dissection, but that self- 
division is a normal though unusual mode of reproduction 
among these animals, as well as in the Protozoa, which 
may also be made to reproduce by artificial division, as 
Ehrenberg cut an infusorian into several pieces, each frag- 
ment becoming a perfect individual. 
