histology of astraeids 103 



Ectoderm. 



The ectoderm forms the entire outer hning of the soft parts, 

 i.e. of the oral-discs, tentacles, column-walls of polyps (where 

 it is termed the calicoblastic layer), and edge-zones and 

 coenosarc (the ectoderm of their outer wall is a continuation 

 of the oral-disc ectoderm, while that of their inner wall is 

 a continuation of the calicoblastic layer). 



In the oral-disq and outer wall of the edge-zone (figs. 1 and 2) 

 the ectoderm has an even free surface which, in sections, 

 is often seen to be covered with mucous secretion and is of 

 more or less uniform thickness. It has a thin free border con- 

 taining fine vertical striae and is provided with short cilia. 

 Elongated nuclei are aggregated somewhat along the middle 

 of the ectoderm ; smaller round ones which are less numerous 

 lie more or less scattered in its inner half. Both mucous and 

 granular vacuoles are present, the former being more abundant 

 than the latter, and a varying number of nematocysts also 

 occur in it. Outwardly diverging tracts are sometimes visible 

 in the protoplasm between the vacuoles. Fibrils are continuous 

 between the ectoderm and middle lamina. At the base of the 

 ectoderm is a finely granular stratum in which a faint network 

 can be discerned which is probably the result of intercrossing 

 of the basal processes of the nuclei of the ectoderm and the 

 fibrils which pass into it from the middle lamina. This stratum 

 has been usually regarded as nervous, but, on renewed examina- 

 tion, no such nerve elements as those described by the Hertwigs 

 in Actinians could be found in it. In the tentacles (fig. 3 and 

 25, fig. 44) the ectoderm is greatly thickened at intervals to 

 form the batteries in which nuclei are considerably increased 

 in number, the elongated ones lying in the upper half of the 

 batteries, smaller oval and round nuclei in the lower half. 

 The granular stratum at the base of the tentacular ectoderm 

 is thicker than at the base of the oral-disc ectoderm. 



The calicoblastic ectoderm (figs. 10, 13-15) is very thin except 

 where the column-wall processes are being formed, and has 

 a somewhat irregular outline and granular protoplasm, the 



