10 Mr. J. E. Duerden on 



endodermal type and weak. The muscle-cells line simple or 

 slightly branched mesogloeal plaitings, which extend from a 

 little below the base of the outermost tentacles to near the 

 commencement of the vesicles, i. e. along the capitular portion 

 of the column. 



Tentacles (PI. 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 tliose 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 

 mesogloea. The mesogloea is thick compared with other 

 regions. The endoderm has numerous zooxantlielUe, and an 

 oblique muscle occurs on smaller mesogloeal plaitings. The 

 endodermal cells vary much in length, the whole layer 

 presenting a very irregular internal boundary. The portion 

 figured is from one of tiie specimens showing an ectodermal 

 reticular or nervous layer very distinctly. 



Disk. — The disk is an extremely thin structure, showing 

 no important characters. 



(Esophagus. — The ectoderm of the oesophagus is much 

 folded in both longitudinal and transverse sections, but no 

 oesophageal 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 zooxanthellee. A 

 weak endodermal muscle is met with. 



Mesenteries. — Tlie mesenteries are in numerous pairs, but, 

 owing to the flatness of the specimens when retracted and 

 the crowded condition of the coelenteron, 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 oesophageal 

 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 coelenteron. Mesenterial filaments, continuous with 

 the ectoderm of the oesophagus and containing nematocysts. 



