IV 



PHYLUM CCELENTERATA 



191 



being invested with ordinary ciliated cells (c.). In virtue of 

 possessing both stinging-capsules and gland-cells, the mesenteric 

 filaments perform a double function. The animal is very voracious, 

 and is able to capture and swallow small Fishes, Molluscs, Sea- 

 urchins, &c. The prey is partly paralysed, before ingestion, by the 

 nematocysts of the tentacles, but the process is completed, after 

 swallowing, by those of the mesenteric filaments. Then as the 

 captured animal lies in the stomach, the edges of the filaments 

 come into close contact with one another and practically surround 



rite 



FIG. 141. Transverse section of mesenteric filament of Sagartia. c. ciliated cells; gl. gland- 

 cells ; -iitc. nematocysts. (After Hertwig.) 



it, pouring out, at the same time, a digestive juice secreted by their 

 gland-cells. 



The muscles described above consist partly of spindle-shaped 

 nucleated fibres, and partly of muscle-processes, like those of 

 Hydra : the latter occur chiefly in the transverse muscular layer of 

 the tentacles and are endodermal, the longitudinal layer is formed 

 of distinct fibres of ectodermal origin : the great muscles of the 

 mesenteries are of course endodermal. Although always derived 

 either from the ectoderm or endoderm, many of the muscle-fibres 

 of Tealia undergo a remarkable change of position by becoming 

 sunk in the mesoglcea, and thus appearing to belong to that 

 layer (Fig. 139 /. m.). This fact is significant from the circum- 



