MADREPORARIAN CORALS 33 



In the families of corals belonging to Group A additional 

 pairs of mesenteries are added to the primaries, but these 

 are unilateral pairs, the two members of a pair being close 

 together on the same side of the stomodaeum.. These uni- 

 lateral pairs of mesenteries are called the Secondary mesen- 

 teries or " Metacnemes." In the diagram (Fig. 7) that has 

 been drawn to illustrate this arrangement eighteen unilateral 

 pairs — metacnemes — have been drawn, but it must be noted 

 that in most of the corals belonging to Group A there are 

 more than eighteen metacnemes, and in many cases a 

 very large number. There is one point 

 also which is of particular interest 

 and importance in the arrangement 

 of these mesenteries. The pairs of 

 metacnemes appear in the spaces be- 

 tween the lateral protocnemes, but no 

 metacnemes are ever found in this 

 group in the spaces between the direct- 

 ive mesenteries. 



The result of this arrangement is that Fig. 7.— Diagram of 



1 J 1 1 J 1 j_ 1, the mesenteries of the 



whereas the lateral protocnemes may be Astraeid polyp Manicina,, 

 separated from one another in a large showing eighteen pairs 



, , J 1 r • £ of metacnemes. II 1-3. 



polyp by a great number of pairs of nii-6. After Duerden. 

 metacnemes, the directive mesenteries 



always stand side by side and can be recognised as the 

 directives throughout life.. 



But this is not the only character by which the directive 

 mesenteries can be recognised. 



One of the functions of the mesenteries is to support 

 the important longitudinal muscles which cause the retrac- 

 tion of the polyps, and the position of these muscles can 

 be seen in a transverse section of a polyp as a series of 

 ridges on one surface only of the mesenteries. In the cases 

 of the directive mesenteries these ridges are on the surfaces 

 opposed to each other, that is to say, they face outwards 

 (see diagram 6, III-IV), whereas in the other mesenteries 

 the muscle ridges face inwards (see diagrams 6 and 7). 



This feature of the directive mesenteries is of some im- 

 portance in the study of the anatomy of the Astraeid corals 



D 



