﻿MORPHOLOGY OF CORAL POLYPS 



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maintained ; one region may be somewhat in advance of, or may lag 

 behind, its normal development. 



The sequence thus outlined in the briefest manner is sufficient to 

 show that the development of the mesenteries in coral polyps is 

 bilateral, and takes place in stages from one extremity to the other. 

 The radial symmetry, characteristic of the adult polyp, is thus derived 

 from primitively bilateral organs, which appear in an antero-posterior 

 succession. Moreover, each cycle represents a separate period of 

 growth, as compared with the successive growth in one direction of 

 ordinarv segmented animals. 



Fig 16.— Transverse section through the stomodceal region of a bioral polyp 

 of Cladocora arbuscitla, showing the character and arrangement of the mesen- 

 teries in a late stage of fissiparous gemmation. The plane of separation is 

 entoccelic, and each polyp exhibits nearly perfect cyclic hexamerism, with two 

 pairs of directives (d). The figure should be compared with figures 14 and 

 15 representing true fission in corals. 



The first two cycles of tentacles (prototentacles) generally arise a 

 cycle at a time, either simultaneously or one following the other. The 

 later tentacles are developed in an order in correlation with that of 

 the mesenteries, sometimes entoccelic and exoccelic members appear- 

 ing together. In the process of growth the exoccelic members are 

 always relegated to the outermost cycles, in a manner first estab- 

 lished by Lacaze-Duthiers for actinians ; only the entoccelic tentacles 

 are of any ordinal value. Siderastrea radians (Pallas) is excep- 

 tional in that the exoccelic tentacles appear in advance of the 

 entoccelic. 



