50 SIDERASTREA RADIANS. 



The septa are therefore related to the mesenteries in the following 

 manner : 



The six primary septa are inclosed within the entocceles of the primary 

 cycle of six pairs of complete mesenteries, the six secondary septa within the 

 entocceles of the second cycle of mesenteries, the five tertiary septa within the 

 entocceles of the third cycle, while the members of the outermost or quater- 

 nary cycle of septa are all exoccelic, one between each two pairs of mesenteries. 

 The primary, secondary, and tertiary septa are all entosepta ; the members 

 of the fourth cycle are exosepta. Further, the number of exosepta equals 

 the sum of the entosepta, and the number of cycles of septa is one more than 

 the number of cycles of mesenteries. Considering the septa as entosepta and 

 exosepta, the true septal formula is 6, 6, x, 12 + x, where x may be any 

 number from one to twelve. The variations in the number of septa within 

 the different calices are in pairs an entoseptum and an exoseptum and affect 

 only the members of the third cycle and their corresponding exosepta. These 

 two cycles always show a corresponding variation which is not explicable on 

 the usual supposition that the cycles of septa are developed in the order of 

 their importance. The order of appearance of the septa and the relationships 

 of the entosepta and exosepta will be discussed more fully in connection with 

 the development of the septa in larval polyps. 



It will be shown that morphologically the exosepta are to be regarded 

 not so much as a separate cycle, appearing after the others are established, 

 but rather as the bifurcated continuations of the original cycle of exosepta, or 

 perhaps as new formations arising along with the entosepta which they 

 inclose ; each entoseptum has an exoseptum corresponding with it, which is 

 formed nearly at the same time. Bxosepta are found to arise along with 

 each cycle of entosepta or even with each individual entoseptum ; but as new 

 cycles of entosepta are formed they are, as it were, shifted outwardly, so as 

 always to constitute the outermost cycle. Until the adult condition is reached 

 the exosepta are but temporary predecessors of the permanent entosepta. 

 The same relationship is found to hold between the entotentacles and the 

 exotentacles. 



The above relationship between the cycles or orders of septa and mesen- 

 teries holds for most species of corals which have been examined. In certain 

 forms the exosepta are absent, e. g., Pectinia, Manidna, when the cycles of 

 septa correspond in number with the cycles of mesenteries. 



Where the hexameral cyclic sequence of a corallite is not completed it 

 is preferable to speak of the exosepta merely as exosepta or as septa of the 

 outermost cycle, not as a third or a fourth cycle ; for ontogenetically some 



