10 Mr. J. E. Duerden on 
endodermal type and weak. The muscle-cells line simple or 
slightly branched mesoglceal plaitings, which extend from a 
little below the base of the outermost tentacles to near the 
commencement of the vesicles, ¢. e. along the capitular portion 
of the column. 
Tentacles (Pl. I. fig. 4).—The ectoderm of the tentacles 
is a thick layer, with numerous closely packed nematocysts, 
uniformly arranged in a zone at right angles to the surface. 
They are smaller than those in the vesicles, and all show 
the spiral thread distinctly. Below the nematocyst band 
is a zone of small nuclei. The ectodermal muscle-cells are 
strong and arranged on well-developed plaitings of the 
mesoglea. The mesogloea is thick compared with other 
regions. ‘The endoderm has numerous zooxanthelle, and an 
oblique muscle occurs on smaller mesoglceal plaitings. The 
endodermal cells vary much in length, the whole layer 
presenting a very irregular internal boundary. ‘The portion 
figured is from one of the specimens showing an ectodermal 
reticular or nervous layer very distinctly. 
Disk.—The disk is an extremely thin structure, showing 
no important characters. 
Gsophagus.—The ectoderm of the cesophagus is much 
folded in both longitudinal and transverse sections, but no 
cesophageal groove is indicated. Abundant deeply staining 
nuclei are arranged a little below the surface. It is richly 
ciliated all round; medium-sized nematocysts crowd the 
layer, and pigment-granules occur in the deeper parts ; a weak 
ectodermal muscle is present. The mesogloea and endoderm 
are very thin, and not folded to the same degree as is the 
ectoderm. ‘The endoderm contains many zooxanthelle. A 
weak endodermal muscle is met with. 
Mesenteries.—The mesenteries are in numerous pairs, but, 
owing to the flatness of the specimens when retracted and 
the crowded condition of the ccelenteron, their arrangement 
cannot always be readily made out. They are somewhat vari- 
able in number, and in the alternation of perfect and imper- 
fect pairs. In one specimen eight pairs of perfect mesenteries 
were present, of which only one pair were directives. In 
another example twenty pairs were counted in the cesophageal 
region. ‘The retractor muscle on the face of the mesenteries 
extends for some distance from the body-wall, and the meso- 
gloea is thrown into long narrow plaits to support it. The 
muscle extends completely round the smaller imperfect 
mesenteries. The mesenteries branch much below, nearly 
filling the ccelenteron. Mesenterial filaments, continuous with 
the ectoderm of the cesophagus and containing nematocysts, 
