PHYLUM COELENTERATA 



141 



ciliated groove, the siphonoglyphe, through which water is constantly 

 carried into the gastrovascular cavity for respiratory purposes. The 

 gastrovascular cavity is divided into radially arranged compart- 

 ments by the primary septa or mesenteries which extend from the 

 wall of the gullet to the inside of the body wall. The primary septa 

 in the axis of and extending parallel with the siphonoglyphes are 

 called directives. At the basal end these cavities are continuous 



Fig. 65. — Sea anemone, Metridium inarginatum, showing external features. 



with the main central cavity. Between the primary septa are sec- 

 ondaries which do not quite reach the wall of the gullet, hence their 

 medial ends are free in the cavity. Between these and the pri- 

 maries are some tertiary septa which are still shorter ajid also 

 attached to the inner surface of the body wall. A quarternary set 

 is represented by mere ridges on the inner surface of the wall and 

 is interspersed among the others. There is a band of muscle run- 

 ning vertically on the face of each septum next to the muscle on 

 the adjacent septa of the same rank. Below the gullet the mesentery 

 has secretory filaments which in turn bear long, threadlike acontia. 



