112 THE EVOLUTION OF THE METAZOA 



epithelia which have been given various names and which have 

 been variously interpreted (indifferent, interstitial, mesenchy- 

 mal, mesogloeal cells). These are frequently hidden between 

 the somewhat dehiscent basal ends either of the one or of the 

 other, or, finally, of the two epithelium layers. Neither during 

 their ontogenies, nor in their fully developed state do these 

 cells represent a compact layer or an epithelial organisation,, 

 and so the name mesoderm which is frequently used in this 

 connection is wrong. I have therefore proposed a new name 

 instead of the former: mesohyl. 



The Regressive Development of the Intermediate Layer 



in Cnidaria 



I must emphasize, w^hen we first make a brief study of the 

 mesohyl, that it is most developed in the Anthozoa where it 

 often contains even muscle cells. This layer can be 

 found well developed in the tentacles of anthopolyps. In the 

 Scyphozoa and in the Hydrozoa it has almost completely dis- 

 appeared and it is only occasionally revived by the actively 

 moving cnidocytes (Had^i 1907, 1911 b, c). In the Cnidaria 

 among others a jelly-like hyaline substance which plays an 

 important role is secreted into this middle layer (Fig. 21). 

 This so-called mesogloeal layer has usually been wrongly in- 

 terpreted as a limit between the entodermal and ectodermal 

 epithelia. Whatever occurs on one side of this layer has been 

 attributed to the entoderm, and whatever appears on the other 

 side has been attributed to the ectoderm, and in this no ex- 

 ception has been made even in connection with gonocytes. 

 Attempts have even been made (Hert\^dg) to use this as a basis 

 for a systematic division of Cnidaria into Ectocarpa and Endo- 

 carpa. We really get the impression that Nature herself had 

 tried to make it easier for zoologists to differentiate between 

 the ts^^o body layers which alone have been supposed to exist 

 (Diblastica, according to Ray Lankester). 



