118 THE EVOLUTION OF THE METAZOA 



to be called ectodermal, even when the taeniolae of Scypho- 

 polyps (the retrograded sarcosepta) were found to be built of 

 an entodermal epithelium, together with some "mesenchyme". 

 Ontogenetically, however, these retractors are formed from the 

 basis of cutaneous infundibula which lie in the oral disc and 

 which are therefore of the "ectodermal origin". These septal 

 retractors have lost their function during the formation of 

 medusae and they have therefore disappeared. On the other 

 hand, we frequently find in Scyphomedusae the layer of longi- 

 tudinally placed muscular fibres to be progressively developed 

 and speciaUzed, in agreement with their swimming type of 

 movement. As this was the case in anthopolyps we also find 

 here the function of the layer of muscular fibres considerably 

 increased, which has led to fold formation in this layer. 

 It is not rare for "mountains" of folds to iDecome separated, 

 obviously secondarily, from the skin epithelium and it is in 

 this way that they develop into the mesogloeal muscles. The 

 outer layer of muscular fibres shows a particularly strong de- 

 velopment under the oral disc of Scyphopolyps. Here it has 

 been given a special function: it helps to open the oral orifice 

 in order to make it possible for the predator to swallow its 

 prey, and for this reason the arrangement of these fibres is 

 radially symmetric. I pointed out, some time ago (Hadzi, 1907) 

 that these muscular fibres can be found cross -striated as early 

 as in Scyphopolyps, a fact which has been little noticed, but which 

 I have found, however, to be of considerable importance in 

 view of the fact that the subumbrellar muscles, which are so 

 important for the swimming abiUty of these animals, had been 

 evolved in both medusoid forms (Scypho- and Hydro-medusae) 

 out of this layer of muscular fibres. In those Scyphomedusae 

 which have reached a larger size these muscles have become 

 progressively widely differentiated (radial, coronal, delta 

 muscles). 



In Hydropolyps that have become very small (there are a few 

 species only that have become secondarily solitary and whose 

 size is therefore slightly larger) the subcutaneous layer of 



