ADULT COLONY. 31 



triangular or wedge-shaped structures, with the narrow apex towards the 

 mesogloea. 



The function of the desmocytes or desmoidal processes, so closely asso- 

 ciated with the mesogloea, is considered to be that of attaching the soft 

 polypal tissues to the hard corallum, and they are always most numerous 

 along the line of union of the mesenteries with the basal wall. As the 

 mesenteries in Siderastrea are attached to the skeletogenic tissues only 

 in the uppermost part of the polyp, desmocytes are developed somewhat 

 sparingly ; furthermore, they are rarely found over regions where the 

 mesenteries are not attached. In such areas a broad calicoblast layer 

 usually intervenes between the mesogloea and the skeleton. Where the 

 skeletogenic tissues, including the mesenteries, are perforated by the synap- 

 ticula, desmocytes are usually present along the line of attachment of the 

 mesentery. 



Bourne (1899) has worked out in a very masterly manner the origin of 

 the desmoidal processes from the single calicoblast cells. These cells, which 

 are strictly the desmocytes, later become connected with the mesogloea, and 

 the structure takes on a feebly striated character. Although a careful search 

 has been made in sections of Siderastrea prepared in various ways, I have 

 found no certain stages in the formation of the desmoidal processes. Cells 

 of various forms occur within the calicoblast layer in regions where desmoidal 

 processes are already present, or may be expected, but none which could be 

 determined with certainty as desmocytes. Anyone familiar with the details 

 of madreporarian histology will appreciate the credit due to Bourne for work- 

 ing out the development of these structures with such a degree of complete- 

 ness. In their fully formed condition they are undoubtedly continuations of 

 the mesogloea, and it is remarkable that they should have originated inde- 

 pendently and later come into fusion with the middle layer. Gardiner states 

 (1902, p. 138) that in the polyps of Flabellum rubrum the first appearance of 

 any desrnocyte could be seen in a granular mass of protoplasm against the 

 corallum, to which, from the first, it seemed to be attached. Subsequently, 

 by growth inwards, it joins the structureless lamella, which may be thickened 

 so as to meet it. 



The actual calicoblast Ia3^er of S. radians, like the endoderm, is at first 

 very narrow, and in the growing areas of the skeleton shows no evidence of 

 cell limitations. The cytoplasm appears continuous, non-granular, and with 

 or without mucous spaces, the whole staining a bright yellow in picric acid. 

 The nuclei are nearly as numerous as in the endoderm, and are large and 

 finely granular. The margin towards the skeleton is usually irregular in 



