THE ANATOMY OP PENTASTOMUM TERETIUSOFLUM. 27 



structure is concerned, the cells of the head, hook, and parietal 

 glands are not to be distinguished from one another. 



The cells have the same fundamental structure as was first 

 described by Leuckart. Each is of large size (fig. 52), and has 

 a distinct nucleus, which sometimes shows a limiting membrane, 

 but often has the appearance of a darkly stained mass of 

 indefinite outline, which almost merges into the surrounding 

 protoplasm. The latter shows a reticulation very similar to 

 that of the cells to be described in connection with the vasa 

 deferentia in the male, and the ducts leading from the recepta- 

 cula in the female. Very frequently these cells may be 

 arranged in groups of four or more when they are closely ap- 

 posed. Around a central spot where the cells are all in con- 

 tact with one another is a light, unstained, somewhat circular 

 area, into which radiate irregular lines of little (when stained 

 with borax carmine) yellowish masses. From the central spot 

 passes off, as figured by Leuckart, a fine thread-like strand, 

 which is in reality, as described by Stiles in P. proboscideum, 

 a hollow tube. The latter in some preparations, where the 

 parts of the gland are more or less separated from each other, 

 and the whole is not so compressed as is usually the case, can 

 be clearly seen to run to the main duct. They have all the 

 appearance of being minute tubes continuous with the chitin- 

 ous lining of the main duct, and doubtless serve to carry to the 

 latter the secretion of these groups of cells. Numbers of the 

 cells forming the gland are apparently independent of each 

 other, though they are all more or less closely packed together. 

 There is nowhere any trace of a definite arrangement of these 

 cells into layers. 



The main duct will be dealt with after the description of the 

 head-gland has been given, 



(2) Head- gland (= anterior part of hook-gland of Hoyle 

 in P. protelis, and hook-gland of Stiles in P. probosci- 

 deum). — The cells forming the head-gland are directly con- 

 tinuous with those of the hook-gland, and also with the parietal 

 cells, to be afterwards described. In structure the three are 

 identical. 



