On the Anatomy of an Arenaceous Polyzoon. 5 



with any degree of certainty the existence of nuclei in this 

 layer, but there can be no doubt that it is composed of cells 

 which secrete the chitinous wall of the tube. Very often 

 this chitinous wall is roughened on its inner surface by 

 minute, sharp prominences and ridges (Fig. 10, p.), which 

 mark the points where secretion is most actively taking 

 place. There can be no doubt that the epithelium lining 

 the internodes is a direct continuation of the coelomic 

 epithelium of the polypides, and it appears to be the only 

 organic connection between the different polypides of the 

 colony. 



The Polypide. 



The structure of the Polypide is essentially the same in 

 both species. As much of it as can be made out in the 

 living animal is represented in Fig. 1, while a more extensive 

 view, such as is represented in Fig. 12, can only be obtained 

 by teasing out the polypides in the manner already 

 described. 



The lophophore (Fig. 6) is of course circular. From its 

 margin spring from 10 or 12 (in C. wilsoni) to 14 (^in G. 

 concretum) tentacles. Within the circle of tentacles ar-e the 

 mouth (oj, placed somewhat excentrically, and the nerve- 

 ganglion (n.g.), lying to one side of the mouth. The anus is 

 outside the circle of tentacles, and is so placed that it lies 

 in a line with the mouth and nerve-ganglion (Fig. 1 2). 



The tentacles are hollow cylinders closed at their distal 

 extremities. The wall of the cylinder, as already pointed out 

 by Allman in the case of the fresh-water Polyzoa, is composed 

 of two layers — an inner, apparently structui'eless layer (Fig. 

 7, s. I.), and an outer, epithelial layer. The epithelium does 

 not present the same character all over the surftice of the 

 tentacle. Over the greater part of the surface it is composed 

 of the ordinary, somewhat flattened, nucleated cells (Fig. 7, 

 ep.), but on the in-turned face of each tentacle there are two 

 parallel longitudinal rows of small columnar cells (Fig. 7, c c), 

 each containing a relatively large, deeply-staining nucleus. 



When the tentacles are examined in the living condition 

 each one is seen to possess two rows of vibratile cilia, 

 projecting on each side beyond its margin. Each cilium 

 is nearly as long as the tentacle is thick, and they always 

 move in a perfectly definite and regular manner, although 

 detachments of them are capable of temporarily ceasing 



