HISTOLOGY OF ASTRAEIDS 109 



and (3) nuclei (figs. 1, 11, and 12). The fibres are of two kinds : 

 those which have a wavy appearance and run in various 

 directions but have chiefly a longitudinal and transverse 

 disposition — such fibres appear to be unbranched and are 

 closely cemented together to form the substance of the lamina ; 

 branching fibres which form a loose plexus in the lamina — these 

 are brought to view by carefully staining sections of not more 

 than 6 /^ thickness. The apparently homogeneous appearance 

 of the middle lamina is due to the thinness and close cementing 

 of the fibres. Nuclei are comparatively few and lie scattered in 

 the lamina ; they become evident in tangential or oblique 

 sections through the thicker regions. Each nucleus is oval in 

 shape, containing a conspicuous spot (the nucleolus), and lies 

 in thin finely granular protoplasm from which irregular pro- 

 cesses usually radiate into the substance of the lamina ; not 

 infrequently a narrow clear space can be detected around the 

 protoplasm. In several West Indian species of coral Duerden 

 noted the presence of ' migrant connective-tissue cells, such as 

 occur in the larger x\ctinians ' (9, p. 22). 



The middle lamina is thickened in the mesenteries and is 

 raised on one side into longitudinal pleats whose breadth 

 and thickness vary in the different species (fig. 9). In the 

 stomodaeal region the pleats extend over part of the width of 

 mesenteries to a varying distance from their column-wall 

 attachments, while below the stomodaeum they cover almost 

 the entire mdth of mesenteries. The lamina is usually con- 

 siderably thickened where mesenteries join stomodaea and 

 column-walls. While at the stomodaeal attachment the 

 thickening is restricted to the ridge, at the column-wall inser- 

 tion the thickening usually spreads a short distance into the 

 adjacent middle lamina, these lateral thickenings appearing, 

 in transverse section, like two arms. The middle lamina is 

 thickened to a less extent in the tentacles and oral-disc ; in 

 the former, outer longitudinal pleats are present w'hich are less 

 conspicuous than those of mesenteries. Processes arise from 

 the middle lamina over the entire extent of the column- wall 

 to attach the soft parts to the corallum, and are more numerous 



