On the Anatomy of an Arenaceous Polyzoon. 7 



opening of the gizzard the walls of the stomach thicken, 

 and the single layer of columnar cells gives place to several 

 layers of spherical cells (Fig. 9, s. c), containing yellow- 

 granules. As was the case in the pharynx the walls of the 

 digestive part of the stomach are thicker in certain places 

 than elsewhere, as shown in the figures. 



In Gryptozoon concretum (Fig. 9) the difference between 

 the digestive and non -digestive portions of the stomach is 

 more strongly marked than in G. wilsoni, and it is in the 

 former species alone that 1 have succeeded in detecting the 

 cilia at the upper end, although there is no reason to doubt 

 that they occur also in the latter. It should be noted, that 

 these cilia are also figured by Allman in certain fresh-water 

 forms. 



A rather narrow aperture (Fig. 9, o. i.) places the upper 

 end of the stomach in direct communication with a saccular, 

 very thin-walled intestine (Fig. 12, int.), terminating at the 

 anal opening («.), the position of which, outside the circle of 

 tentacles, has already been indicated. 



The entire alimentary canal, which in all Polyzoa has the 

 form of a loop, is clothed externally by a delicate, closely- 

 fitting, flattened epithelium — the coelomic epithelium (Fig. 9, 

 e/j. ; Fig. ] 2, n. e.), the nuclei of which are plainly discernible 

 over the greater part of its surface. On the intestine, these 

 nuclei are in places elongated in the transverse direction, and 

 this takes place especially in the region just opposite to the 

 nerve ganglion, giving rise to a deceptive appearance of 

 columnar epithelium in this locality. 



To the lower portion of the stomach is attached the 

 funiculus or posterior mesentery (Figs. 11 and 12, fun.) in 

 a perfectly normal manner. 



The muscular system is well developed. There are : — (1) A 

 circle of short retractor muscles (Fig. 1 1 , m.) attached above 

 to the introversible portion of the zooecium. Of these 

 muscles, each of which is composed of a band of simple 

 fibres, I have counted four, and they appear to be all 

 attached at the same level. It is possible that a fifth occurs, 

 but has escaped notice owing to its being hidden behind the 

 others; (2) A broad band of long fibres (Fig. 11, r. m.; Fig. 

 12, 771.) attached above in a semi-circle just beneath the 

 bases of the tentacles, around the margin of the lophophore 

 remote from the anus. These fibres converge below to a 

 point (Fig. 11, a. m.) deep down on the near side o the wall 

 of the zooecium. Tliis band is the great retractor muscle of 



