ADULT COLONY. 25 



The mesenteries extend from above downward for about two-thirds of 

 the total length of the polyp, the younger outer pairs terminating in advance 

 of the older inner pairs. Below they become very short transversely, and 

 are only slightly folded at their free extremity. Except in the uppermost 

 region of the polyp, their peripheral portion is everywhere perforated by the 

 synapticular bars. Further, they are connected outwardly with the wall of 

 the polyp only in the upper part of their course ; the connection breaks down 

 below, and the organs are seen in transverse sections with both their inner 

 and outer ends free. Thus at an early stage the peripheral portion of the 

 mesentery undergoes degeneration or resorption much in advance of the more 

 central part. The mesenteries, in fact, occupy only the middle and upper 

 part of the gastro-vascular cavity ; the proximal and peripheral regions are 

 practically destitute of them. 



The mesenterial mesoglcea is usually narrow, but there is much varia- 

 tion in the different polyps as to its thickness and hence that of the mesentery 

 as a whole. In the upper region the mesogloea presents vertical folds for 

 the support of the retractor muscles (plate 7, fig. 41). Generally the fold- 

 ings are diffuse and somewhat complicated, and extend along the entire 

 face of the mesentery ; in other cases they are stronger and more restricted 

 to the middle area. The oblique musculature is very weak throughout, but 

 occasionally the fibrils can be seen cut obliquely in transverse sections. As 

 would be expected, no basilar or parieto-basilar muscles occur. The mesen- 

 terial endoderm is everywhere richly supplied with Zooxanthellse, and also 

 with clear gland cells and others containing large granules. As described 

 in connection with the endoderm of the column wall and disc (p. 22), what 

 seems to be an intercellular system of mucous spaces occurs in the deeper 

 parts of the epithelium on both mesenterial face. 



The arrangement of the mesenteries may be studied in more detail. As 

 represented on plate 6, fig. 34, and also in the diagrammatic figure on page 26, 

 the number of pairs within each of the six primary systems or sextants 

 varies from one to three. The members of a pair can be easily distinguished 

 by their similarity of size and by the retractor muscles being on the faces 

 turned towards each other. In each of the two ventral systems only one 

 pair of mesenteries (n) is developed, while in the middle and dorsal systems 

 two pairs (u, in) are present, with the exception of the right middle system, 

 which contains three pairs. Where only one pair occurs, it represents one of 

 the six pairs of first-cycle metacnemes (n), and this cycle is complete in all the 

 mature polyps examined. Where two pairs are present, one pair (in) is shorter 

 than the other and is a member of the third cycle. It will be observed that 



