16 G. HERBEET FOWLER. 



arrangement of the latter is well shown. Into the central cavity project the 

 six septa. In two of the innermost ring of polyps, the axial and abaxial 

 septa have fused into the septal columella-plate. 



Pig. 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 costai, 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 

 supporting the stomatodseum. (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. 



YiG. 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 stomatodseum are shown the two openings 

 of the canals of mesenteries 7, 9, 11 ; and below the stomatodaum 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 costee have pierced the body 

 wall on account of its shrinkage. This drawing shows the arrangement of the 

 external longitudinal canals, and their connections between the spikes of the 

 costae. (Camera lucida.) 



Fig. 8. — Diagram of the various forms and conditions of the mesenteries 

 in a polyp of Type A. Those numbered 1, 3, 5, 8, 10, 12 are unmodified and 

 normal. The others, 2, 4, 6, 7, 9, 11, are modified in all the polyps of this 

 type ; they are from camera lucida drawings of the same mesentery at difi^erent 

 heights. The arrows and Roman numerals in Fig. 8 show the planes in which 

 the successive sections are taken. — 2 shows the endodermal swelling, and the 

 upper opening of the canal ; 6 shows the lower opening ; 9 is below the 

 stomatodseum, and bears a filament ; and in 11 no trace of the modification 

 remains, the mesentery being normal, and similar to those of Type B. 



Fig. 9. — Transverse section of a modified mesentery, passing through both 

 arms of the canal. 



Fig. 10. — Transverse section of the corallum of a branch of M. asp era. 



