302 DR. J- E, DUERDEN ON THE 



the former the strands sometimes appear slightly reticular in character. Small nuclei 

 occvir among the fibrils, and are regarded as the nuclei of ganglion-cells. 



The same fibrillar layer is still better developed in some polyps of B. antilliensls. It 

 may occur on both the ectodermal and endodermal faces of the mesoglcea, and what 

 seem to be the continuations among the ectodermal cells are clearly seen (1891, pi. i. 

 figs. 3, 4). In Protcmthea, Carlgren (1893, pi. iii. figs. 4, 5) represents the mesogloeal 

 continuations, both on the ectodermal and endodermal aspects, as constituting a nearly 

 compact zone, while in the preparations of Bunodeopsis they appear as very djstinct 

 coarser strands, rather widely separated one from another. 



The nerve-layer in Bunodeopsis is, as a rule, very narrow in sections, and appears 

 made up of interlacing fibrils, some seen longitudinally and others in section. In Carl- 

 gren's figures of Protanthea the layer is represented by minute dots only, indicating the 

 cut ends of the fibrils. Such an appearance also occurs in the stomodseal ectoderm of 

 Bunodeopsis (fig. 7, nr.l.). 



The mesogloea of the column-wall is thin throughout, and both its inner and outer 

 surfaces are smooth, or very slightly plaited to afford additional support for the ecto- 

 dermal and endodermal muscu.latures. 



The endoderm is broad in the lower region of the polyp, where, as a rule, the cells 

 contain many Zooxanthellse ; but the layer becomes very narrow above, as the symbiotic 

 algse are either absent or very rare, and the cells are then more charged with proto- 

 plasmic contents. The endodermal free surface is everywhere feebly ciliated. 



The circular endodermal muscle occurs throughout the column-wall, usually supported 

 on very delicate mesogloeal plaitings. A slight increased development takes place 

 towards the upper extremity, but never to such a degree as to give rise to w^hat may be 

 regarded as a sphincter muscle (fig. 3, end.m.). A nerve-layer is usually seen, situated at 

 some distance from the mesoglcBa, and a little better developed than in the ectoderm. 



The evaginations (figs. 4 & 5, ev.) are simple, hollow, rounded outgrowths of the lower 

 part of the column-wall, and in transverse sections are seen to be both entocoelic and 

 exocoelic in position, though without any regular alternation. In sections the walls at 

 their thickest are about "065 mm. across. The cavity communicates by a large aperture 

 with that of the ccelenteron. Both the ectoderm and endoderm have undergone certain 

 histological modifications compared with the character of the layers in the column-wall 

 generally. The ectoderm (fig. 5) is broad, somewhat strongly ciliated, and a very 

 distinct zone of oval nuclei occurs just within the outer margin, w^hile smaller, rounded 

 nuclei occur here and there throughout the thickness of the layer. The characteristic 

 elements, how^ever, are the large, thick-walled nematocysts (neni.), which occur in some 

 abundance in the outer area of the evaginations. Seen in sections, the nematocysts 

 exhibit few or no structural details, but appear as smooth-walled, nearly homogeneous 

 bodies, irregularly distributed among the supporting cells. The deeper examples more 

 especially are arranged at varying angles witli the surface of the ectoderm, but the outer 

 are practically parallel with the supporting cells. The former are probably individuals 

 in process of development and are strongly curved, while when they assume their 



