156 THE INVERTEBRATA 



violently at intervals until the communicating strands snap and one by- 

 one the ephyrae swim away. The ephyra develops into the adult by 

 the filling up of the adradial notches in the margins as well as by the 

 growth of the bell as a whole. The mesogloea increases enormously 

 in thickness, causing the two layers of the endoderm to come to- 

 gether as a solid lamella except where the canals occur. The mesen- 

 teries lose their attachment and cease to exist as partitions with the 

 collapse of the enteron, but their position is marked by the gastric 

 filaments. The basal part of the scyphistoma remains and grows 

 new tentacles, and after a resting period as a hydratuba may strobilate 

 again. 



A Be 



Fig. 130. Strobilization of Aurelia aurita. From Sars. A, Hydratuba on 

 stolon which is creeping on a Laminaria. The stolon is forming new buds at 

 I and 2. B, Later stage or scyphistoma, x 4. The strobilization has begun. 

 C, Strobilization further advanced, x 6. D, Free-swimming ephyra stage, 

 showing first appearance of unbranched adradial canals, x 7-5, seen from 

 below. E, The same seen in profile, x 7*5. 



The Rhizostomeae are a division of the Scyphomedusae in which 

 the four lips around the mouth are vastly developed and folded, and 

 the central mouth itself is narrowed and in a number of forms en- 

 tirely closed. It is replaced by thousands of small ** sucking mouths " 

 which lie along the course of the closed-in grooves of the lips. These 

 lips now constitute organs of external digestion. Small copepods and 

 even fish are enclosed by the lips, digested and the fluid absorbed 

 through the "sucking mouths" which are too small to admit solid 

 particles of any size. The young medusa of Rhizostoma still has a 

 central mouth, but in the adult, e.g. Pilema here figured (Fig. 131), 



