ADULT COLONY. 27 



The radial length of the mesenteries varies greatly in passing trans- 

 verse sections in review from the oral to the aboral extremity of a polyp. In 

 an expanded polyp the complete mesenteries extend from the colnmn wall, 

 then across the disc, and finally unite with the stomodasum, down which 

 they pass ; the members of the incomplete cycles, on the other hand, stretch 

 from the column wall only partly across the disc, and terminate at a greater 

 or less distance from the middle of the polyp. When the level of the corallum 

 is reached, all the mesenteries begin to lose their peripheral connection 

 with the polypal wall, and in transverse sections hang freely, unconnected 

 either peripherall}- or centrally, except where pierced by a synapticulum. 

 Even at the level of the stomodasum in partly retracted polyps the mesen- 

 teries are greatly shortened transversely, so that the peripheral parts of 

 the septal loculi are empty (plate 6, fig. 34). Where the loculi are inter- 

 rupted transversely by a synapticulum, a mesentery will sometimes stretch 

 from an inner chamber of the loculus to the next chamber, but in no instance, 

 as shown on plate 6, fig. 34, is one found to extend right to the periphery 

 of an interseptal loculus. Instead of this, most of the peripheral chambers 

 are empty, the mesenteries having become resorbed. The organs have a 

 much less radial or peripheral extent on plate 7, fig. 38, resorption having 

 been continued further than at the level represented by plate 6, fig. 34. 

 Mesenteries are wholly absent on plate 7, fig. 39, except in one or two 

 isolated instances. The stages prove conclusively that the mesenteries 

 extend but a ver}^ short vertical distance within the region of the synapti- 

 cula, but are longer centrally. The same fact is also well shown on plate 6, 

 fig. 36, representing the upper part of a tangential section of a polyp. 



The descriptions given by Bourne (1887, 1893) of the anatomy of the 

 synapticulate coral Fungia indicate similar degeneration phenomena of the 

 mesenteries within the region of the synapticula in this genus. At first both 

 the primary and secondary mesenteries extend to the very base of the coral- 

 lum, but afterwards they are confined to the upper moiety of the calice, 

 though in Fungia no dissepiments are formed which cut off the polyp 

 from the basal plate. Bourne's figs. 10, 13, and 15 in his first paper show 

 that the mesenteries very rarely extend along the interseptal loculi all the 

 way from one synapticular perforation to another, but in practically all 

 the canals long or short remnants of the mesenteries are adherent to the 

 synapticular wall, and serve for the attachment of separate bundles of the 

 longitudinal muscles. In these cases the middle part of the mesentery within 

 each chamber has been resorbed, leaving only its two extremities. In Sider- 

 asirea there is nothing corresponding to the mesenterial muscle bundles in 



