10 THE OCEANIC HYDROZOA. 



analogous to those cellulai- elements which coat the gastric cavity and the intestines of 

 so many of the lower animals." In his Biphijcs gracilis, Gegenbaur states that the " cells "' 

 possess a distinct double contour and a nucleus, neither of which have I ever observed. 

 " There can be no question," he adds, " that these elements must be regarded as glandular 

 cells; and their ultimate fate is in favour of this view. For we find between them empty 

 depressions, with sharply defined edges, which correspond exactly in form and size with 

 those in which the cells in question are imbedded. These depressions can be hardly 

 anything else than the spaces formerly occupied by such glandular cells, which have burst 

 and emptied their secretion." 



Kolliker (p. 26) gives a still more definite account and figure of the structure of the vilU 

 and vacuoles in Athori/bia. He considers them to be " glandular sacs of the simplest kind," 

 also that they are " open sacs, hned with cells," and inclines to the opinion that their function 

 is hepatic. 



Leuckart takes the same view of the cavities in the villi of the Plii/sopJwridm as myself, 

 regarding them not as cells, but as vacuoles (Z. N. K., p. 68). 



Vogt (p. 102) appears also to consider the cavities of the villi in Prai/a to be vacuoles 

 and not cells ; and he gives an account of an experiment, which would be well worth repeating. 

 " Having mixed indigo in the water of a vessel containing a lively Praya, I saw after 

 some time that the digestive cavities were streaked with blue, the colouring matter being- 

 detained by the villi [hourrelets) ; and I convinced myself by microscopic examination that the 

 colouring granules existed only in the celloeform spaces, which are nothing else than shallow 

 depressions or widely open glandular sacs." 



Without by any means denying the possibility that the vacuoles (for such mere 

 excavations full of fiuid I must confess they always appeared to me to be) contain a special 

 secretion, I am inclined to think that the villous eminences in which they are imbedded have 

 other functions. I once observed a half-digested mass in the stomach of an Atlioryhia, all the 

 villi in the neighbourhood of which were much elongated, and thrust into it. The ends 

 of these villi contained fewer thread-cells than usual, while many thread-cells were scattered 

 through the mass of food. Is it not possible that when the living prey is introduced into the 

 gastric cavity, its struggles may be restrained and cut short, not only by the mechanical 

 application of the elongated villi, but by the shooting out of the threads of the numerous 

 thread-cells with v.hich they are provided r 



AUman^ has described structures corresponding very closely with these vacuolated villi 

 in Cordylopliora, and it is therefore probable that something of the same kind will be found in 

 other Hydrozoa. But, so far as I know at present, the only structures in the Lucernariada 

 to be compared with the villi are those solid tentacular filaments, with vacuolated axes, 

 which project from the cndoderm into the stomach or into the somatic cavity. They are, 

 like the vilh, covered at their extremities with abundant thread-cells. 



The villi and vacuoles are confined to the gastric division of the polypite. The walls of 

 the distal or buccal division are thin and smooth, but richly ciliated internally. 



1 "Anat. et Pliys. of Cordylophora," 'Phil. Trans.,' 1853. 



