34 



CORALS 



in particular, because the polyps of some of the corals 

 belonging to this family appear to be entirely devoid of 

 directive mesenteries, and this fact can only be explained 

 by their method of asexual reproduction. 



The different kinds of asexual reproduction in the 

 Madreporarian polyps may be arranged in two categories : 

 reproduction by gemmation or budding and reproduction 

 by fission or division into two. Reproduction by gemmation 

 may be intercalicinal when the buds arise from the coenosarc 

 between the calices, epicalicinal when they arise from the 

 outer wall of the calyx, or intracalicinal. 

 The buds produced by all these methods of 

 gemmation always show throughout life 

 two pairs of directive mesenteries. In 

 reproduction by fission in the Astraeid 

 corals the mouth and stomodaeum con- 

 strict in the middle to form two mouths 

 and two stomodaea (Fig. 8), and when the 

 body wall of the polyp follows suit the 

 metacnemes II 2 take up a position opposite 

 to the directive mesenteries 1 4 and 1 5 in 

 the resultant polyps, and thus each of 

 these daughter polyps has only one pair 

 of directive mesenteries. \\'hen these 

 daughter polyps are large enough to divide 

 they each give rise to one with one pair of directive 

 mesenteries and one with none. 



And thus it comes about, by a continuation of this 

 process, that in a large colony of Astraeids the directive 

 mesenteries appear to be absent in all the polyps, although 

 theoretically there should be somewhere in the colony two 

 polyps each with one pair. 



In the families belonging to Group B, the directive 

 mesenteries and the other protocnemes are formed as in 

 the corals of (iroup A, but, as they are not succeeded by 

 any series of metacnemes, the normal number of mesenteries 

 in a full-grown polyp is twelve. 



In the process of fission in the genera Madrepora and 

 Porites belonging to this group a new set of twelve mesen- 



FiG. 8. — Diagram 

 to show a stage in 

 the division by fis- 

 sion of the Astraeid 

 polyp Manicina. I 3 

 and 1 4 the directive 

 mesenteries. After 

 Duerden. 



