II CNIDARIA— STRATIFICATION OF BODY 103 



In the hermaphrodite Ctenophora we meet with them on or in the 

 walls of the meridional vessels, in such a way that on the one wall of 

 the meridional vessel the male elements lie, and on the opposite wall 

 the female elements. These sexual glands are so arranged that in 

 each of the 8 regions of the body, separated by the meridians, there 

 are either 2 testes or 2 ovaries. The ripe sexual products fall into 

 the meridional vessels, and through the gastro-calnal system reach the 

 stomach and oesophagus and pass out through the mouth. 



In the Cnidaria no special ways of transmission for the sexual 

 products, and no special copulatory organs, are developed. 



Concerning the origin of the sexual products we may say, generally, that in very 

 many Hydrozoa they are developed out of the ectoderm, but in the Scyphraoa out of 

 the endoderm. Observers are not yet agreed about the origin of the sexual products 

 in the Otetiophora. 



Since in the one form, the Sydroids, the sexual products come from the ectoderm, 

 and in a related form fi-om the endoderm, too great significance should not be 

 attached to the place of their origin. 



XI. The " Stratifleation " of the Cnidarian Body. 



In the lowest Cnidaria the body during life consists of two layers 

 of epithelium separated by a supporting lamella ; these two layers are 

 similar to the two epithelial germinal layers of the gastrula larva. 

 The musculature is formed by processes of the ordinary epithelial cells. 

 Only the sexual products arise and continue to lie imbedded in the 

 epithelium. 



As the complication of the organism increases, there is a tendency 

 for certain tissues and organs to detach themselves from the epithelium 

 and to take up a position beneath it. This tendency is shown by the 

 various- tissues and organs approximately in the following order: — 



1. The sexual organs, which in the lowest Cnidaria are already 

 subepithelial, and in the higher Cnidaria come to lie altogether or 

 partly in the jelly. 



2. Connective tissue elements, which immigrate into the gelatinous 

 supporting membrane. 



3. The musculature, whose elements first arrange themselves into 

 a subepithelial muscle layer, and then also move (partially at any rate) 

 into the jelly. 



4. The tendency to take up a position deep in the body affects the 

 nervous tissue far less. In consequence of the inseparable connection 

 of the nervous system, on the one hand with the sensory organs, which 

 in accordance with their functions must remain at the surface, and on 

 the other hand with the musculature which tends to sink below it, this 

 system takes up an intermediate position. 



We observe, then, in the Cnidaria the progressive development of 

 an intermediate layer between the outer body epithelium and the 

 inner intestinal epithelium, this intermediate layer being formed of 



