SUBCLASS II 



HEXACORALLA 



91 



are developed in an order in correlation with that of the mesenteries, some- 

 times entocoelic and exocoelic members appearing together. In the process 

 of growth the exocoelic members are always relegated to the outermost 

 cycles, in a manner first established by Lacaze-Duthiers for actinians ; only 

 the entocoelic tentacles are of any ordinary value. Siderastrea radians (Pallas) 

 is exceptional in that the exocoelic tentacles appear in advance of the 

 entocoelic. 



B 



Fig. 127. 



Growth stages of larval polyps in Siderastrea radians. Three rlia,L,Taiiiiiiatir figures illustrating the manner 

 of appearance of the six mesenteries (A-C) constituting the second cycle. The mesenteries arise in unilateral 

 pairs within corresponding exocoelie chambers on each side of the polyp. At tirst («) a i)air api)ears within the 

 dorso-lateral exocoele on each side ; shortly after (h) a sinular pair arises within each middle exocoele ; then (c) 

 a pair within each ventro-lateral exocoele. For a long time the pairs retain a difference in size, corresponding 

 with their order of appearance (after Duerden). 



The skeleton never appears until after fixation of the larva. It makes its 

 first appearance in the form of minute plates or granules, as an ectoplastic pro- 

 duct of the ectodermal cells (calicoblasts) of the base. A flat, circular, basal 

 plate is formed by the union of these, and may later become i:)roduced upward 

 at the edge as the epitheca, while from its inner or polypal surface the septa 

 begin to appear as vertical upgrowths formed within invaginations of the 

 basal disk of the polyp. The skeletal cup first formed is known as the 

 prototheca. 



