THE ANATOMY OF PENTASTOMUM TBBETIUSCULUM. 13 



(fig. 28). The two groups of fibres, unlike those described in 

 other species, such as P. tsenioides, do not cross each other, 

 nor do they typically pass from one segment to another. 

 Leuckart^ has described two bands on each side, one passing 

 downwards and forwards, the other downwards and backwards; 

 whilst Hoyle,- in P. protelis, finds that only the former of 

 these two groups is present. In P. teretiusculum two 

 groups are present, but only in rare instances can any passage 

 of fibres from one segment to another be detected. The fibres 

 pass, in two parallel bundles, in a dorso-ventral direction, and 

 are inserted almost entirely into the wall of the anterior part 

 of the annulus ; occasionally an odd fibre, or a terminal branch 

 of one, belonging to the posterior group runs backwards into 

 the very anterior part of the annulus next behind. In the 

 space between the oblique fibres and the body-wall are placed 

 groups of parietal gland-cells (fig. 12, Pa. gl.). 



Hook-muscles (fig. 39). — There is very considerable diffi- 

 culty in rightly determining the number and arrangement 

 of these, which accounts for the discrepancies in the various 

 accounts given. The figure drawn represents the results 

 obtained from dissections, and from the examination of some 

 eight series of consecutive sections cut in various directions. 

 Whilst it agrees fairly well with the description given by 

 Stiles,^ it differs from his in one or two important respects, but 

 to avoid confusion I have as far as possible followed the names 

 given by him. 



In connection with the hooks there are four special portions 

 which serve for the attachment of the muscles. These are (1) 

 the upper angle of the base of the hook, (2) the lower angle 

 of the base of the hook, (3) the basal joint, and (4) the 

 special chitinous thickening of the wall in the lower part of 

 the invagination, containing the base of the hook with which 

 it is united (fig. 38, 1—4). 



The muscles may be clearly divided into two sets : (1) those 



1 Loc. cit., p, 41, Taf. i, fig. 11. 



^ Loc. cit., p. 172, pi. xxvii, iig. 10. 



' Loc. cit., p. 143, Taf. viii, figs. 30, 33. 



