HISTOLOGY OP ASTRAEIDS 107 



colony, the ectoderm of the free surface is continuous with the 

 stomodaeal hning at frequent intervals. The inner lining of the 

 stomodaeum of the larva is not raised into ridges at the mesen- 

 terial attachments, hence both grooves and sulci are absent ; 

 its nuclei are not so crowded together nor so slender as in the 

 polyp, and are arranged along the middle of the layer wdiose 

 protoplasm is conspicuously granular and opaque above the 

 nuclei, while below them it is vacuolated and translucent ; 

 the condition of the stomodaeal lining of the larva is on the 

 whole intermediate between that of the oral-disc ectoderm 

 and the stomodaeal ridges of the polyp. 



Mesenterial Filaments (figs. 7, 8, and 18). 



The epithelium of mesenterial filaments has essentially 

 the same structure as the inner lining of the stomodaeum, 

 the median lobe being similar to the stomodaeal ridge and the 

 ventro-lateral tracts to the parts between the ridges. The 

 median grooves of most of the stomodaeal ridges are continued 

 to some distance along the middle of the straight regions of 

 their corresponding mesenterial filaments, the cilia in the 

 grooves being longer than those over the rest of the filaments. 

 A transverse section through a principal filament just below 

 the stomodaeum, as shown in fig. 6, bears striking resemblance 

 to Ashworth's figure of a transverse section through a dorsal 

 mesenterial filament of Xenia Hicksoni (3, fig. 19). 

 Granular vacuoles are frequently present in the straight region 

 of the filaments. Nematocysts are few in the straight region, 

 numerous in the convoluted parts, where they often become 

 massed together. Each of the ventro-lateral tracts of a fila- 

 ment is organically continuous with the mesenterial endoderm 

 on its side. Filament-epithelium is present on subsidiary 

 mesenteries (except on the very narrow ones) along the greater 

 part of their length, but is rudimentary in their upper half 

 or one-third, where it contains a few aggregated nuclei or 

 is sometimes entirely absent. Subsidiary filaments are smaller 

 in transverse section than principal filaments. Mesenterial 

 filaments of the free-swimming larva (whether of principal or 



