REPORT ON CORALS — HYDROCORALLIN^. 45 



cates by tubular offsets with the axial cavities of the tentacles, and at the periphery 

 of its base it becomes continuous with the cavities of four large canals. These canals 

 subdivide almost immediately into smaller trunks which anastomose with the general 

 ccenosarcal meshwork. 



The gastrozooids are structurally composed of the same number of layers as the 

 dactylozooids. The ectoderm forms on these zooids a somewhat thinner layer than 

 on the dactylozooids. Definite cell structure was not made out in it. It is, however, 

 full of nuclei, and is no doubt definitely cellular in the living condition. It is not, as 

 in the case of the dactylozooids, thickly beset with nematocysts, but contains very 

 few of these bodies (PI. X. figs. 1 and 5). 



On the inner surface of the ectoderm, in combination with the basement membrane, 

 occurs a muscular layer which is very highly developed. The layer is composed of a 

 series of longitudinally disposed muscular slips, which are set side by side with narrow 

 interspaces, so as to form a thick layer (PI. X. fig. 6). This layer is extremely 

 thick and dense towards the base of the zooid, as will be seen from Plate III. M, and 

 becomes gradually thinner and less conspicuous towards the hypostome. The 

 muscular slips are stout and closely set towards the base of the zooid, and are pro- 

 minent objects in transverse sections of it in that region (PI. X. fig. 5), whilst they 

 are widely separate and fine and far less numerous towards the upper regions of the 

 zooid (PI. X. fig. 1, M), where little is to be seen but the transparent basement 

 membrane. The muscular slips are composed of very distinctly differentiated cells 

 which have mostly a fusiform shape (PI. X. fig. 8), with the tails of the cells 

 usually somewhat bent. Many cells are found to occur amongst the mass which are 

 apparently in the act of division, two fusiform bodies being connected together by a 

 string, or broad mass, of protoplasm. Such cells are so numerous that possibly a 

 considerable proportion of the muscular elements remain permanently in this com- 

 pound condition. The cells are closely fitted together side by side to form the mus- 

 cular slips which, where most developed, have a breadth of three or four cells (PL X. 

 fig. 7). 



The longitudinal muscular slips pass from the basis of the zooids to spread out 

 beneath the ectoderm of the four main canals of the ccenosarc in which the cavities of 

 the zooids terminate interiorly. 



Fused with the muscular layer occurs, as in the dactylozooids, a continuous layer of 

 membrane. This basement membrane is transparent, and the only structure which I 

 have seen in it is a striation transverse to the longer axis of the zooids, which, as 

 already stated in reference to the dactylozooids, I at first believed to give evidence of 

 the existence of circular muscular fibres in the zooids. Such fibres I have, however, 

 been unable to discover on closer examination. 



Beneath the membranous layer lies the endoderm. This is composed, towards the 



