ADULT COLONY. 49 



three septa occur in a S3'stem the middle member belongs to the second order, 

 and the two lateral ones are exoccelic; where five septa are present, one 

 belongs to the second order, another to the third, and the three alternating 

 with these to the exoccelic cycle ; in the rarer instances in which seven 

 septa are developed the middle member is a second-order septum, the two 

 next in size are tertiary septa, and the four alternating septa belong to the 

 fourth or exoccelic cycle. The members of the outermost cycle fuse in an 

 intennipted manner by their inner ends with the members of the second 

 and third cycles, but not with those of the primary cycle ; the tertiary septa 

 in their turn unite with the secondary. 



The actual ordinal relationships of the septa can be best understood 

 when taken in connection with the mesenteries, as revealed in transverse 

 sections of the polyp. 



Plate 6, fig. 34, represents a transverse section from the lower stomo- 

 daeal region of a partly retracted decalcified polyp. The drawing was made 

 with the help of a camera lucida, but in the process of decalcification and the 

 preparation of the section slight displacements of the lamellae have taken 

 place. Still the general relationships remain as in the living condition, and 

 for all practical purposes the invaginations of the polypal wall correspond 

 with the septa which have been dissolved away. Fig. 2, p. 13, is a diagram- 

 matic representation of the septa of the same polyp, the synapticula and 

 spinous projections being omitted for the sake of clearness. 



In the transverse section (plate 6, fig. 34) a large septal invagination 

 (entosepta, I, 11, iii) occurs within the interspace inclosed by the two 

 members of each pair of mesenteries, and a smaller one in the interspace 

 between every two adjacent pairs (exosepta, x). The entosepta inclosed by 

 the two pairs of directive mesenteries are known as directive septa, but in 

 the corallite itself there is nothing to distinguish these from the other 

 primary entosepta. They indicate the dorso- ventral (postero-anterior, sulculo- 

 sulcar) axis of the corallite, and can only be determined with certainty when 

 in association with the mesenteries. 



The six interspaces between the six primary mesenterial pairs contain 

 a variable number of pairs of mesenteries : the two ventral systems each 

 contain one mesenterial pair (il), the right middle interspace three pairs 

 (ill, II, III), and the remainder two pairs each (in, 11). The six pairs of 

 secondary mesenteries (11) are easily distinguished from the tertiary mesen- 

 teries (ill) by their greater size and more central position. Of the latter 

 there are only five pairs in this particular polyp instead of twelve, as 

 considerations of hexameral symmetry would suggest. 



