INTERNAL ANATOMY OF BDELLA. 5lS 



transverse section. It is invested by a loose, outer coat of mucus-like substance, bearing 

 little groups of darkly staining dots irregularly scattered ; within this it has a thick, 

 dense tunic which is irregular in surface ; the interior is provided with what in longi- 

 tudinal section (fig. 39) appear to be a series of fine crinkled lamella3, which are fixed by 

 their outer edges to the inner side of the wall of tlie gland and project far into the 

 lumen, only leaving about one-third in the middle free; these appai"ent lamellae are 

 about 1 n thick and about 5 ,u apart — their projection into the lumen is about "02 to 

 "03 mm. If, however, a transverse section, or, indeed, even a horizontal one of the gland 

 be cut, these apparent Itimcllse will still be found presenting an almost similar appearance 

 of being parallel laniellte on edge projecting into the lumen. The explanation of this is 

 that if they are to be regarded as lamellfe perpendicular to the length of the organ then 

 they are joined by numerous short lamella3, slightly thinner than the principal ones and 

 running obliquely at various angles; probably, however, they are rather to be regarded 

 as a closely-packed mass of long columnar cells, of very small diameter, and of somewhat 

 singular and irregular form, being triangular, rhomboid, or pentagonal in section, but 

 oftenest triangular, and which are never, in any specimens which I have seen, closed 

 at the inner ends, neither ha\^e I ever detected any nuclei to these cells, if cells they be, 

 nor any cell-contents ; the walls only persist, and this although I have examined numerous 

 specimens of various species caught at almost all times of year (except mid-winter, when 

 I cannot get any) and of various ages. 



When the gland is cut open, and the open ends of these cells, if cells they be, are looked 

 at from wdthin, they present the irregular reticulated appearance shown at fig. 23 ; if 

 the microscope be focussed a little within the mouths of the cells then a singular 

 appearance is obtained, very like the well-known tissue of the rush composed of stellate 

 cells ; of course this can only be an accidental resemblance produced by crossing or 

 sloping lines. 



The central lumen, beyond the lamellae, is often, but not always, filled with a quantity 

 of secretion very similar in appearance to that of the great mucous glands ; in hai*dened 

 specimens it is often a rod-like nmss, and occasionally shows on the exterior of the rod 

 casts of the parts of the inside of tlie cells which abut on the lumen of the organ, as if 

 the secretion had been pressed into the mouths of the cells. It is possible that this 

 matter has entered the gland from the exterior and is not secreted, in which case the 

 organ would not be a gland at all ; the very dense character of the wall looks a little like 

 it, but I do not think it probable, particularly as there are not any constrictor muscles 

 wdiich would enable the organ to act for expulsory purposes. 



The Air-chambers. — These organs (figs. 18, 22, acg) arc two paired organs, one on each 

 side of the body, which vary considerably in form in different specimens and species. 

 They are far more developed in B. Basteri than in any other species which I have 

 examined ; in that creature each one is a chamber of somewhat irregular form, round in 

 transverse section, but much longer than the diameter ; it averages somewhere about 

 •15 mm. long and about '05 mm. to -08 mm. in diameter, according to what part is 

 measured, for the organ is considerably thicker at one end than the other. The whole 

 organ is lined by, indeed it would be almost correct to say formed by, a strong chitinous 

 intima, which, however, is transparent and only slightly brownish in colour; it is not 



