﻿IOO SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 47 



large polyps a centimeter or so, being cut off from the skeleton below 

 by transverse dissepiments. 



The three coelenterate layers lining the skeleton, and by which the 

 skeleton is produced, have each undergone certain modifications com- 

 pared with their character in the column and oral disk. In most 

 instances the endoderm becomes greatly thickened and vacuolated in 

 passing from the upper to the lower regions of the polyp, so that 

 proximally it greatly reduces the polypal cavity, the mesenteries 

 being also wanting in this region. 



The mesoglcea, as elsewhere throughout the polyp, is a thin homo- 

 geneous lamella, but in certain areas it is produced into conical or 

 wedge-shaped processes, striated toward their extremity, and coming 

 into direct contact with the skeleton. These have recently been 

 shown by Bourne to take their origin from modified ectodermal cells, 

 which he terms dcsmocytcs, and the desmoidal processes are con- 

 sidered to serve as a means of attachment of the polyp to the corallum. 

 They are best developed along the line of union of the mesenteries 

 with the skeletotrophic tissues, and where skeletal formation is not in 

 progress. 



The basal ectoderm is the true skeletogenic epithelium, and varies 

 greatly in character according as it overlies a region of active skeletal 

 formation, or an older region where growth is stationary. At the 

 apex of branches, and the edge of growing septa, the layer is broad 

 and highly protoplasmic, while the individual cells, termed calico- 

 blasts, have either distinct walls or are fused into a common mass or 

 coenocyte. In the non-growing areas the skeletogenic layer is usually 

 extremely narrow, being scarcely recognizable in sections of decalci- 

 fied material. The calicoblasts are usually in an active state in the 

 deepest or most proximal part of the polyp, as it is here that the cal- 

 careous dissepiments are continually being formed as the polyp 

 grows upward. 



In verv carefully decalcified material, from rapidly growing 

 regions, a homogeneous skeletal matrix or ground substance remains 

 in the space formerly occupied by the skeleton. It resembles the 

 mesoglcea in its behavior toward reagents, though with the form 

 and detailed microscopic appearance of the skeletal fibers. It is 

 thus manifest that the calcareous skeleton, though formed ex- 

 ternally to the polyp, is laid down within a colloidal matrix produced 

 by the skeletogenic tissues. After decalcification of the older parts 

 of the skeleton there is little evidence of this organic matrix except 

 immediately next the calicoblast layer. Here the matrix is usually 



