﻿DUERDEN] morphology of coral polyps 99 



of the terminal lobe is the same, but in certain corals it undergoes 

 a remarkable modification, constituting what must be regarded as a 

 special glandular organ. For a limited pari of its course the filamenl 

 becomes greatly enlarged, and all the cells, with the exception of the 

 supporting cells, are charged with a finely granular secretion, yellow- 

 ish in color. Sometimes the glandular differentiation may extend so 

 as to include part of the mesenterial epithelium adjacent to the 

 filament. 



In the lower part of their course the mesenteries and filaments 

 may become greatly convoluted, and, when much disturbed, the living 

 polyps have the peculiar power of extruding the filament along with 

 the part of the mesentery to which it is affixed; they can, however, 

 he again indrawn as the polyps recover. Sometimes the filaments are 

 forced out in such quantities as to give a ragged appearance to 

 the surface of a colony. The extrusions take place through the oral 

 aperture or any part of the column wall or disk; in the latter case 

 minute pores are made in the polypal wall, hut after the mesenteries 

 and filaments are indrawn the punctures are healed, and the wall then 

 affords no indication of their former presence. 



The phenomenon of extrusion of the filaments is in some ways 

 comparable with the emission of acontia through cinclidal pores which 

 takes place in certain anemones (sagartids) ; but here the acontia are 

 sent out as threads distinct from the mesenteries, and the cinclides 

 are permanent apertures, whereas, in corals, a part of the mesentery 

 accompanies the filament, and the openings are temporary and may 

 be produced at any part of the polypal wall. 



SKELETOGENIC TISSUES 



All the researches on the development of corals, as well as on 

 their adult relationships, serve to demonstrate that the entire coral 

 skeleton is an ectoplastic product of the basal disk, that is, it is 

 formed altogether external to the polyp itself. On decalcification the 

 polyps remain as perfect organisms, their walls intact throughout, 

 intricate infoldings of the lower part representing the spaces formerly 

 occupied by the skeleton. All the radial (cost?e, septa) and tangential 

 (theca) elements of the corallites correspond with so many infoldings 

 of the basal part of the polyp, which have been produced pari passu 

 with the deposition of calcareous matter. 



Considering the colony as a whole, the polyps themselves are alto- 

 gether superficial, however thick may be the coral stock. The soft 

 tissues extend onlv a few millimeters within the skeletal mass, or in 



