98 INVERrEBRATE ZOOLOGY 



Digestive System. — The mouth is usually oval or slit-liko and 

 leads through an inturned tube of ectoderm, called the esophagus, 

 into the gastrovascular cavity. Like the mouth, this esophagus 

 is usually compressed so that a sagittal axis is recognizable 

 marking off a biradial symmetry. In some instances, one or two 

 grooves run the length of the esophagus at the poles of the sagittal 

 axis. These grooves (x, Figs. 56, 57) are termed the siphono- 

 glyphes. The esophagus extends only part way from the oral 

 toward the pedal surface. It is held in position by the primary 

 septa (Fig. 55, /), folds of entoderm and mesoglea which extend 

 from the body wall inward to meet the esophagus and thus at the 

 oral extremity completely divide the gastrovascular cavity into 

 a number of pockets or chambers. 



Number and Arrangement of Septa. — Since each chamber of 

 the gastrovascular cavity continues beyond the body proper into 

 a tentacle, there is usually a direct relation between the number 

 of chambers and the number of tentacles. In addition to the 

 primary septa, many others reach only part way toward the 

 esophagus. These are of varying lengths and on the basis of 

 their length and order of development are termed secondary, 

 tertiary, and so forth (Fig. 55, II, III). The number of primary 

 mesenteries is of considerable importance in distinguishing the 

 various orders of the Anthozoa. In the Alcyonaria (Octocoralla), 

 there are eight septa. In practically all representatives of this 

 order, the muscle ridges of all septa are directed toward the same 

 pole of the sagittal axis. Edwardsia is an exception to this rule 

 and marks a step in transition from the Alcyonarian to the 

 Zoantharian (Hexacorallan) type, for in Edwardsia the position 

 of the muscles on one pair of septa at one pole of the sagittal 

 axis is reversed. The Zoantharia usually have one pair of 

 primary septa at each pole of the sagittal axis and two lateral 

 pairs on each side of the body. The two pairs lying in the 

 main axis (Fig. 55 ?•) are called the directives. The muscle 

 ridges on these face outward, while in all the remaining pairs 

 the ridges of one pair face each other. In development, the 

 primary septa are the first to appear. Between these, come the 

 secondary and later the tertiary septa. With the increase in 

 number of septa, there is usually a corresponding increase in 

 number of tentacles, but this correlation is not absolute. 



Other Organs. — In addition to muscles, the gonads, mesen- 

 terial filaments, and acontia are important structures associated 



