SCYPHOZOA 179 



eight notched lobes being formed, four of which are interradial and 

 four perradial. In each notch there is a short tentacle and this becomes 

 a tentaculocyst. Each lobe is provided also with two short lateral 

 tentacles, but these disappear. A prolongation of the gastric cavity 

 into each lobe indicates the beginning of the branched perradial and 

 interradial canals, and at a little later stage the adradial canals also 

 appear (Fig. 134 D). The gastric filaments are also seen as four pairs 

 in the interradial mesenteries. 



The Scyphistoma is the name given to the segmented body and each 

 of the segments is an Ephyra larva (Figs. 130 D, 134). They lie upon 

 each other like a pile of saucers, connected, however, by strands of 

 tissue in which run the muscles of the interradial mesenteries con- 

 tinuous throughout the pile of individuals. These muscles contract 

 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 enorm- 

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

 together 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. 



The life history of the sessile form may thus be summarized. The 

 hydratuba feeds and buds in the summer, continues to feed and stores 

 food in the autumn but ceases to bud, strobilates in the winter, grows 

 new tentacles in the spring and feeds and buds again. In this the 

 Scyphomedusae show features in common with the life history of 

 the hydroid colonies and the freshwater Hydra. 



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 of that and other forms, e.g. Pilema 

 here figured (Fig. 135), it is entirely closed. Cassiopeia is a semi- 

 sedentary form, which lies with its exumbrellar surface upwards on 

 the mud of mangrove swamps. The bell pulsates gently and brings in a 

 constant stream of plankton organisms which are seized by the lips. 



