34 SIDERASTREA RADIANS. 



even in the most roughly decalcified specimens, and no definite cell limita- 

 tions could be distinguished ; but over the greater part of the corallum it is 

 an extremely thin, finely granular layer, and as the edges of the septa are 

 approached the layer thickens, nuclei become more frequent, and tend to 

 exhibit a definite network. Where secretion may be supposed to be going 

 on most actively the layer is more hyaline, elsewhere it is granular. The 

 protoplasm is much vacuolated, as in Siderastrea, except where secretory 

 activity prevails. 



Where decalcification has been carefully carried out, a layer of the 

 organic matrix within which the skeleton is deposited usually remains 

 behind, and is closely adherent to the calicoblast layer, as shown at the left 

 side of plate 8, fig. 45. The matrix usually appears perfectly homogeneous, 

 very variable in width, and recalls the mesogloea in its behavior towards 

 stains. Bourne (1899) f un( l such a layer in many of the forms examined 

 by him, and regards it as a fine membrane " limiting membrane " separat- 

 ing the calicoblasts from the corallum, and comparable with the sheath which 

 incloses the spicules of the Alcyonaria. It is evidently a secretion of the 

 ectodermal layer, and at the apical points of rapidly growing corals, such as 

 Madrepora^ I have found the deposit to be continuous throughout the whole 

 thickness of the corallum, and presenting just beyond its border the fibrous 

 scale-like appearance of the early skeleton (1902, p. 484). It is manifest that 

 the mass should be regarded as a homogeneous, mesogloea-like matrix within 

 which the minute calcareous crystals forming the skeleton are laid down, to 

 be compared with the matrix which Bourne has demonstrated for the skeletal 

 spicules of Alcyonaria. As a continuous structure it early disappears, and 

 in the older parts of the skeleton is represented merely by the fine organic 

 particles remaining after decalcification. Only under the most favorable 

 circumstances is it present as a continuous mass in sections of the newest 

 formed parts. Usually all that remains of it after decalcification is the deli- 

 cate membrane bordering the calicoblast layer on its skeletal aspect. When 

 of sufficient thickness to have contained skeletal fibers, now dissolved away, 

 this membrane appears fibrous, but immediately bordering the calicoblast 

 layer it is homogeneous. 



SEPTAL INV AGINATIONS, INTERSEPTAL LOCULI, AND GASTRO-CCELOMIC CAVITY. 



The septal invaginations (refoulements septaux of Delage & Herouard) 

 are the vertical, somewhat wedge-shaped infoldings or upgrowths of the 

 basal wall which in the living condition cover both sides of the septa, 

 and have been produced pari passu with them. Their presence results in 



