146 McMURRICH. [Vol. IV. 



In such sections the mesenteries present a very different struc- 

 ture from what is found lower down. 



The endoderm in such a section (Fig. 8) resembles very closely 

 in structure that of the column wall, but lower down, in sections 

 which pass through the column wall and the stomatodaeum, it 

 is much lower, very granular, and without any trace of cell out- 

 lines. In the gonophoric region (Fig. 9) it again becomes high, 

 higher even than in the uppermost region. The protoplasm of 

 the cells is crowded towards their free extremities, which take 

 the carmine stain, and in this region the nuclei are most abun- 

 dant ; towards the mesogloea, however, the cells form a network 

 (Fig. 10), the meshes of which are mainly occupied by a sub- 

 stance which does not stain. Slight traces of a granular sub- 

 stance are also present, and in some regions there are large 

 numbers of apparently homogeneous spherical bodies (Fig. 10), 

 which do stain somewhat deeply, and which vary considerably in 

 size. It is possible that they may be nuclei, but I am rather 

 inclined to think from their homogeneity and varying size that 

 they are food particles. They occur also in the endoderm of 

 other regions. 



It is possible that the network is formed by branching and 

 anastomosing processes of mesogloea, but such an origin for it 

 could not be made out. If it should be of this nature, it would 

 be in accord to a certain extent with what Danielssen has 

 described in his C. borealis. Delicate lamellae having a wavy 

 outline project from the mesogloea of the mesenteries into the 

 epithelium : upon their surface are arranged both longitudinal 

 and transverse muscle fibres. No such arrangement occurs in 

 C. Americamcs, nor apparently in C. fnembmnaceus and solitariiis, 

 but, as stated, the network, if mesogloeal, might be regarded as 

 representing it. The granular substance which lies along the 

 fibres composing the network no doubt corresponds to the mus- 

 cle fibres described by Danielssen, these, like the pinnate lamellae, 

 being "ausserst diinne." Maceration preparations of C. Ameri- 

 canus failed to show the presence of muscle fibres in the meshes 

 of the network. In the endoderm of the mesenteries, as else- 

 where, no Zooxanthellae are present. 



In the sections which pass through the disc and column wall, 

 the endodermal musculature of the mesenteries is very clearly 

 seen (Fig. 8). The muscle fibres form a single flat layer on 



