TYPE C(ELENTEKA. 



105 



chambers communicating freely with a central space. The 

 mouth does not open directly into the coeleuteron, as in the 

 hydroid polyps, but into a tube, the stomatodceum (stj, lined 

 by ectoderm and communicating freely below with the central 

 coelenteric space. Certain of the mesenteries in their upper 

 portions are attached to the endoderuiic surface of the stoma- 

 todreum, but below its lower end all have free edges. Along 

 this free edge there runs, in most of the mesenteries, a cylindri- 



me 



FIG. 56. Metridium margina- 

 tum, LES. 



rm 



FIG. 57. DIAGRAMMATIC TRANS- 

 VERSE SECTION THROUGH Edward- 

 sia IN REGION OF STOMATODCEUM. 

 me = meseulery. 

 rm retractor muscle. 

 si siphonoglyphe. 

 st = stoinalodiEuin. 

 I-IV = mesenteries in the order of 

 their development. 



cal thickening, the mesenterial filament, composed in its lower 

 portion of cnidoblasts and gland cells, and usually longer than 

 the mesentery, on the edge of which it forms a complicated 

 system of coils and twists. In its lower part the filament in 

 some forms separates from the mesentery as a bunch of fine 

 filaments richly provided with nematocysts and capable of 

 being protruded from the mouth or through pores in the 

 body-wall. The upper part of the filament is usually of dif- 

 ferent structure, bearing on each side a baud of elongated 

 ciliated cells whose function it is to produce a circulation of 

 the fluids in the coelenterou. 



The reproductive cells develop in the endoderm of the 



