THE ANATOMY OF ALOTONIUM DIGITATUM. 361 



tion of the intermesenterial space of the coelenteron, is con- 

 tinued into each of the pinnae. In Alcyonium palmatum 

 the ectoderm of the pinnse is thickened in regular patches, which 

 contain batteries of nematocysts. I have seen similar patches 

 on the pinnae of our English species in favorable preparations. 



The mouth varies in appearance very considerably. When 

 fortunate enough to be able to focus the microscope quite 

 vertically down on to a fully expanded polyp, the mouth is seen 

 to be a slit-shaped opening on a shallow ridge in the centre of 

 the flat oral disc. At one end it has a crescentic widening, 

 which indicates the position of the siphonoglyphe lower down 

 in the stomodaeum. When the polyp is slightly retracted, as 

 is usually the case, the true mouth cannot be seen, since the 

 oral disc is depressed in a funnel-shaped manner, forming, as it 

 were, a secondary mouth, which is oval in outline (fig. 6). 



The stomodaeum of Alcyonium digitatum consists of a 

 short, somewhat flattened tube about 1 ram. in length, opening 

 to the exterior by a slit-shaped mouth, and into the coelenteron 

 by an oval aperture. 



On the ventral side there is a siphonoglyphe extending 

 through two thirds of the aboral end (fig. 14). The epi- 

 thelium lining the siphonoglyphe is considerably thicker than 

 it is in other parts of the stomodaeum (see Section IV), and 

 provided with numerous long flagella. 



The epithelium of the stomodaeum other than that of the 

 siphonoglyphe is thrown into a series of six longitudinal folds, 

 which are rather more pronounced at the oral than at the 

 aboral end (PI. 38, fig. 12). 



In this respect the stomodaeum of the polyps of A. digita- 

 tum show a marked contrast with that of the polyps of A. 

 palmatum. In all the sections of the polyps of this species 

 that I have examined the ectoderm of the stomodaeum is quite 

 smooth, as is represented in pi. 1, fig. 2, of my former paper (16). 



The mesenteries of Alcyonium digitatum present no 

 features that require special comment. As in all Alcyonarians, 

 they bear on their ventral faces the large retractor muscles. 

 The details of their histology are given in the next section. 



