TYPE C(ELKNTERA. 



113 



7. Order Hexactiuiae. 



In the Hexactinise the six primary pairs of mesenteries 

 described as occurring in the two preceding orders are again 

 found (Fig- 65, /, D, and D), and in a few forms (Halcampa) 

 may be the only ones present. As a rule, however, a varying 



D 



a 



i 



I.T 



D 



FIG. 65. DIAGRAMMATIC TRANSVERSE SECTION OF AN HEXACTINIAN, Aip- 

 tasia, with only the mesenteries of the first cycle perfect. 



D = directive mesenteries. / mesenteries of the first cycle. 



g = reproductive region of mesentery. // = " " second " 



mf = meseuterial filament. 111= " "third " 



number of secondary pairs develop, each of these appearing 

 in the interval between two primary pairs, so that two cycles 

 of mesenteries (/ and //) may be distinguished. Usually, 

 however, the process of mesentery formation does not stop 

 here, tertiary (///), quaternary, etc., cycles being developed, 

 the pairs of each new cycle appearing in the intervals between 

 the pairs of the cycles already present. Consequently, since 

 there are six primary pairs, the second cycle will consist also 

 of six pairs, the third of twelve, the fourth of twenty-four, and 

 so on. In a few forms, owing to the precocious development 

 of one or two of the secondary pairs on each side, the 

 symmetry becomes converted from an hexanierous one to an 

 octamerous (Aiptasia annulata) or a decamerous one (Tealia). 

 Since the tentacles develop in connection with the spaces 

 between the mesenteries, they are arranged in cycles corre- 



