THE ANATOMY OF THE MADREPORARIA. 15 



Fig. 4. Diagram of a transverse section of a polyp and of part of the 

 branch. The external body wall is shown to be supported 

 on the costae, as its mesoderm and endoderm are continuous 

 with those lying on the outer face of the corallum. The 

 polyp cavity shows at this point twelve mesenteries sup- 

 porting the stomatodaeum. (In nature the mesoderm lies 

 closely apposed to the surface of the corallum, and there is no 

 space between them, such as is introduced into the diagram 

 for clearness.) 



Fig. 5. Transverse section of a portion of the branch, to show the 

 lines of growth running between the canals radially and 

 terminating each in a costa. 



Fig. 6. Diagram of a longitudinal section of a polyp along the dotted 

 line in Fig. 8. The tentacles are omitted, as they were not 

 recognisable in my specimens ; the canal system in the 

 corallum is also omitted. On the left the section passes 

 between the axial septum and mesentery No. 7, and above 

 the polyp down an external longitudinal canal ; on the 

 right, through the abaxial septum and down a costa, of 

 which the echinulations and the canals between them are 

 shown. The numbers indicate the same mesenteries as in 

 Fig. 8. On the mesentery 7 is figured the endodermal 

 swelling, with the bent canal indicated by dotted lines. 

 In the stomatodaeum are shown the two openings of the 

 canals of mesenteries 7, 9, 1 1 ; and below the stomatodaeum 

 the free edges of these three mesenteries alone appear, the 

 others dying out before this plane is reached. The dotted 

 line indicates the junction of theca and septa, and the 

 discontinuous character of the septum (Ab.) is clearly 

 shown. 



Fig. 7. The external body wall viewed from the exterior ; the 

 lighter spots are the places where the echinulations of the 

 costa: have pierced the body wall on account of its shrinkage. 

 This drawing shows the arrangement of the external longi- 

 tudinal canals, and their connections between the spikes of 

 the costse, (< 'amera lucida.) 



