THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 29 



region to open as described by Lohrmann on the papilla, and 

 not ventrally to this as in P. proboscideum, according to 

 Stiles. Of this fact I have satisfied myself after examination 

 of sections of the head of several specimens. 



It will be noticed that there is no separate duct for the 

 head- and hook-glands, nor have I been able, after long search- 

 ing through series of sections cut in various directions, to 

 detect any openings of small ducts from the cells of the head- 

 gland, either into the main duct, which forms a prominent 

 feature as it passes forwards to the papilla of its own side, or 

 into the ducts leading down to the hooks. The latter ducts 

 open, as described by Leuckart, into the invaginations in 

 which the bases of the hooks are placed, and on the ventral 

 side of these (fig. 9, 0. Hk. GL). 



In structure each duct consists of a layer of small cubical 

 cells with distinct nuclei (figs. 24, 27, Hk. Gl. d.), and is lined 

 internally by a thick chitinous layer, whilst the very fine 

 secondary ducts arise irregularly, are usually very difficult to 

 distinguish, and appear to consist simply of a thin chitinous 

 wall. The main duct runs right on through the posterior part 

 of the gland without dividing, and merely receiving along its 

 course these minute ducts, which pass ofi', as described above, 

 from small groups of the cells composing the gland. 



(3) Parietal Cells [Pa.gh), — These consist of irregularly 

 arranged cells, which in the posterior two thirds of the body are 

 not found in the mid-dorsal or ventral lines, and in mature 

 specimens are nearly entirely confined to the space which lies 

 between the oblique muscles and the body-wall (figs. 12 — 23, 

 28). In younger forms and at the very posterior end (fig. 21) 

 they are more largely developed, but are absent in the mid- 

 dorsal and ventral lines. Anteriorly they spread over the 

 whole body-wall internally, and are directly continuous at 

 about the level of the oesophagus with the cells forming the 

 head-gland (fig. 11). As in other forms, there appear to be 

 no special ducts for these cells. 



In addition to the above there are present, as mentioned 

 previously, amongst the muscle-fibres and connective tissue 



