112 



COELENTERATA. 



f 



the two walls occurs the encloderm epithelium is retained in the 

 form of a layer of cells called the vascular or endoderm lamella 

 (Fig. 107). 



In many medusae these concrescent places appear as four small septal unions 

 (C'athamniata, Fig. 98 se). They delimitate four wide radial pouches of the 

 stomach, which however communicate with one another peripherally beyond the 

 unions by a kind of circular canal. 



In the Luccrnaridae and Charybdeidae the septal unions are extended into 

 elongated partitions (Fig. 98, sc), the septa, which delimitate the four radial 

 pouches in nearly their whole extent. But the septa never quite reach 



the umbrella edge, so 

 that there the four 

 canals are connected 

 by a narrow circular 

 canal. The gastric 

 pouches open into the 

 in/ stomach by the gastral 

 ostia (Fig. 99 g.o). 



"When the four 

 places of fusion of 

 the dorsal and ven- 

 tral stomach walls 

 are of considerable 

 extent, the gastral 

 pouches have the 

 form of four narrow 

 radial vessels : this 

 is characteristic of 

 the tetra-radiate 



craspedote medusae. A circular canal connecting them peripherally 



is however retained. 



In many acraspedote medusae there are, beyond and independent of the four 

 septal unions, sixteen lines of adhesion between the dorsal and ventral walls of 

 the gastric pouches. In some cases the four septal unions are not formed, and 

 only the peripheral lines of adhesion appear : in this way the more or less com- 

 plicated form of the peripheral gastrovascular apparatus of the Semostomae and 

 Hhizostomac is introduced. Sometimes the adhesion-lines are narrow and extend 

 up to the edge of the disc (Pelagidae, Cyaneidae), sometimes they are broad and 

 leave a circular canal at the periphery. In the first case broad, blindly-ending 

 gastric pouches are formed ; in the latter the pouches are reduced to narrow 

 vessels which frequently branch peripherally, or even anastomose (Aurclia, 

 Fig. 100, Ehizostomu). 



A similar branching of the radial vessels occurs also in the Craspcdola. The 

 endoderm lamella may become secondarily excavated, and thereby new radial 

 vessels may arise, vessels dichotomously branching in a peripheral direction, or 



Fin. 99. Section through the central stomach of an acraspedote 

 medusa (Periphylla mirabilis). u.g umbrella-jelly ; inf the 

 four funnels ; g.c basal stomach ; /gastral filaments ; g.o gas- 

 tral ostia ; sin circular sinus ; kn septal unions ; s gonads. 



