CNIDARIA 



57 



growth of the gastrostyle, and a general flattening of the bell 

 (Brooks). 



Upon these metamorphoses, above all upon the loss of the 

 solid larval tentacles — of which the perradial are always re- 

 sorbed, while in some forms the interradial are retained 

 alongside the later-developed, hollow, permanent (perradial) 

 ones — is based the metamorphosis of the Geryonidae, accu- 

 rately described by Leuckart, Fr. Muller, and E. Haeckel. 



In Aglaura and Rhopalonema the entoderm is not produced by de- 

 lamination, but like that of the hydroid polyps, since there is formed at 

 first a solid so-called morula-stage destitute of cleavage cavity, the super- 

 ficial cells of which are converted into the ectodermic layer, while those 

 within represent the entoderm (Metschnikoff, No. 12). 



Fig. 23.— Larva of ^ginopsis three days old, with two tentacles (after Metschni- 

 koff, from Balfour's Comparalive Embryology), m, mouth; t, tentacle. 



The development of the Narcomedusce from the egg has 

 become known principally through Metschnikoff (Nos. 12 

 and 13). In ^ginopsis mediterranea the formation of the 

 entoderm is accomplished by multipolar ingression. In the 

 course of cleavage there is no distinct cleavage cavity pro- 

 duced, but from a very early period cells migrate into the 

 interior from any point whatever of the surface, and these 

 constitute the entodermic cell-mass. Since the ectoderm 

 becomes flagellate, there is produced an elongated, rod -like 

 planula, which has almost the a,ppearance of a detached 

 tentacle of a hydroid, for its interior is filled with ento- 

 dermal cells, which are arranged in a single row at either 

 end, being more crowded in the middle portion only. Soon, 

 however, these afterwards bent ends are seen to grow out 



