188 THE EVOLUTION OF THE METAZOA 



resemble closely those that can be observed in Turbellaria and 

 in Ctenophora that have become more and more flat and large, 

 i.e. to the formation of a richly branched system of peripheral 

 pockets and canals which correspond to the general (here 

 medusoid) form of a spheric calotte. A basis for this can be 

 found in the conditions that occur in the corresponding polyp 

 forms. The conditions in the primitive scyphomedusae whose 

 height is greater than their width (e.g. in Cubomedusae) are 

 still very primitive; they are similar to those that exist in 

 scyphopolyps. In these no canals can yet be found, and in 

 particular there is no circular canal along the periphery of 

 their umbrellas. Their central part (stomach) is connected with 

 the gastral pockets by way of four ostia; these are separated 

 from each other b y four broad septa and they do not yet contain a 

 funnel-shaped cavity. The gonads grow out ot the bases of 

 septa into the cavities of the gastral pockets. The peripheral 

 gastral cavities have been developed later, after a further flat- 

 tening and extension of the umbrella; they are due either to 

 a further growth of the intestine, or to the fact that the two 

 gastral walls have grown together locally. There are frequently 

 whole bunches of septal filaments growing into the gastral 

 cavity in those places where septa (taeniolae) had been reduced. 

 It is unnecessary to discuss further details here. 



Additional complications appear in the oral lobes of the 

 most highly developed scyphomedusae (Rhizostomae); in these 

 the original oral openings have grown together while at the 

 same time "suctorial mouths" have been formed at the very 

 numerous fringes of the oral lobes. The intestinal diverticula 

 of tentacles that have become solid in scyphopolyps (as has 

 already been mentioned) can again become hollow in scypho- 

 medusae, or they can completely disappear together with the 

 tentacles while at the same time their function is taken over 

 by the excrescences of the oral lobes. All this development 

 took place while microphagy was strongly advanced. Because 

 of the complications of their gastrovascular system, scypho- 

 medusae have been enabled to grow to considerable dimensions 



