STRUCTURE OF IIYDRA. 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 fibrillae 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. These large cells, 

 from combining the functions of muscle and nerve, are 

 termed " nervo-muscle cells." 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 inf usorian into several pieces, each frag- 

 ment becoming a perfect individual. 



