A NEW SPECIES OF PENTASTOMUM. 179 



about 0'008 mm. in diameter, within which a small nucleolus is generally to be 

 seen ; the protoplasm is finely granular and faintly stained. 



These cells are arranged in groups of 2-5, and very commonly at the point 

 where several cells meet may be observed an oval speck, from which fine radi- 

 ating lines branch out (figs. 9 and 15). The similarity between this appearance 

 and that depicted in some of Leuckart's figures will be at once apparent.*" 



The distribution of these cell-groups next demands attention ; they lie 

 among the smaller mesodermic cells constituting the body-wall, about 

 midway between its inner and outer surfaces, and are disposed in zones, corre- 

 sponding with the annuli of the body, and consequently with the stigmata, none 

 being found in the interannular spaces. It is, in fact, the presence of these cells 

 which causes the swelling of the annular regions of the body and the greater 

 stiffness of the wall in those parts, which has been already alluded to. 



In the annuli, however, these cell-groups are very closely placed, and in a 

 transverse section they seem to form an almost continuous zone ; more than 

 fifty can often be seen in a single circumference. 



The homology of these cells next demands our attention. Their position 

 would seem to show that they correspond to those cells of L. toenioides which 

 Leuckart has called the hook-gland (Hakendrlisen, see Tab. i. fig. 11 of his 

 monograph), and this view is strikingly confirmed by a study of their appear- 

 ance (a/, fig. 15 with Tab. i. fig. 17). The form of the cells is the same, as also 

 their arrangement in acini composed of 2-5 ; the radiating apparatus also, 

 which Leuckart regards as the commencement of the excretory apparatus, is 

 the same in both. But if we admit this hypothesis we are placed on the horns 

 of a dilemma, for we have in the creature before us a structure which is homo- 

 logous with the hook-gland of L. toenioides, and another organ which is beyond 

 all question the homologue of a gland which Leuckart finds in P. proboscAdeum 

 and other species, and which van Beneden has described in his L. Diesingii, 

 and which, again, is declared to be the homologue of the hook-gland of L. 

 tamioides.f Thus we have two separate organs both corresponding to the same 

 organ. It appears to me that at present the only course open to us is to assume 

 that we have here a condition in which there is a double set of organs, one of 

 which is lost in certain forms, while the other disappears in others. This does 

 not seem satisfactory, however, because both structures appear to be very Avell 

 developed, and have by no means the aspect of rudimentary structures. 



In the face of this difficulty, I regret the more that I was unable to trace the 

 course of the excretory apparatus of these peripheral gland cells, even after the 

 most careful scrutiny of the sections. This knowledge might have solved the 

 mystery, but at present it must be left until some future occasion may furnish 

 a supply of better preserved material. 



* Loc. at., Tab. i. fig. 17. f Loc. cit,, p. 67. 



