526 REPORT ON THE XENIIDAE COLLECTED BY DR WILLEY. 



connect together the coelentera and the longitudinal and superficial canals are exceed- 

 ingly numerous, as also are the cords and strands of cells. In X. crassa and X. Novae 

 Bntanniae the longitudinal canals are very well marked and the transverse canals are 

 fewer in number than in X. viridis but still are very numerous (Fig. 8). In all species 

 the mesogloea (except the denser ring round each coelenteron referred to above) is 

 traversed by numerous strands of cells which place all its parts in intimate communi- 

 cation. (See Fig. 8.) 



In sections which pass very obliquely through the ectoderm and mesogloea 

 of polyps of X. viridis small stellate cells, connected with fine fibrils which run 

 inwards to the endoderm, may be seen. This system of fibrils and cells has been 

 more fully described in X. Hicksoni (1899. p. 277), in the account of which it was 

 suggested that these are nervous elements homologous with the similar plexus seen 

 in the base of the ectoderm of Alcyonium and compared by Hicksou with the 

 " Nervenschicht " of the Actiniae (1895, p. 371). 



Ectoderm. 



In the inner or basal portions of many of the ectoderm cells of the tentacles 

 there are muscle fibres which are longitudinal in direction. These are much more 

 highly developed on the oral than on the aboral face. It is owing to their contraction 

 that the tentacles of some of the specimens are somewhat curled inwards over the 

 mouth. The ectoderm of X. membranacea contains numerous nematocysts which are, 

 like those of X. Hicksoni and of most other Alcyonaria, exceedingly small (see p. 513). 



The spicules are similar throughout, being round or oval discs of horny consistency 

 (Figs. 5 and 6). On examining sections or stained preparations of the ectoderm each 

 spicule is seen to be accompanied by the nucleus and remains of the protoplasm of 

 the cell in which it has been formed. 



Endoderm. 



In none of the specimens does the protoplasm of the endoderm cells present the 

 well-marked reticulate structure which was noticed in X. Hicksoni. In A', viridis, 

 in which the endoderm is moderately well preserved, the cells lining the coelenteron 

 and particularly those covering the mesenteries are long, narrow, columnar cells which 

 have a finely granular protoplasm with few or no vacuoles. 



Zooxanthellae are present in the endoderm of the body of the polyp but in 

 comparatively small numbers ; they are more numerous in the endoderm of the 

 tentacles and pinnules, especially in the latter, where they are sometimes so abund- 

 ant that they almost fill up the cavities of the pinnules. 



Gonads. 



Gonads are present in all the species on the edges or sides of the ventral and 

 lateral mesenteries, and are developed from the cells which are present in the 

 mesogloea of these mesenteries near their free edges (see p. 525 and Fig. 1-t). The 

 gonads are most abundant in the portions of the coelentera contained in the upper 

 part of each stem. 



