A NEW SPECIES OF PENTASTOMUM 171 



more nearly circular. A space free from muscles passes down the middle line 

 ventrally, and this space, as we shall see, extends throughout the whole length 

 of the animal. 



The bundles are thus arranged : — About four or five thin band-like portions 

 are placed in the compartments formed by the splitting of the ventral margin 

 of the oblique muscle-layer (fig. 10), and a somewhat thicker one lies on the 

 inner surface of this layer close along its ventral margin. It is these bands 

 which, overlapping each other by their outer margins, produce the appearance 

 of a broad band, seen when the body-wall is examined from within (PI. XXVII. 

 fig. 17, m.JI). 



Passing outwards there are next noticed smaller bands, of somewhat 

 irregular figure in section, lying between the oblique muscles and the body- 

 wall ; and then, after passing the point where the former of these are inserted 

 into the latter, we come to a number (8-12) of rounded bundles which stand 

 out from the body-wall, and would lie free in the body-cavity were they not 

 covered with a thin membrane (ccelomic epithelium ?). 



In Leuckart's* description of these muscles in P. proboscAdeum, he 

 mentions a space free from muscles extending down the dorsal median line of 

 the animal similar to that above mentioned on the ventral aspect ; this state of 

 things certainly did not obtain in the species at present under consideration, 

 the longitudinal bundles succeeded each other quite regularly across the dorsal 

 surface, and there was no thinning out of the mesoderm towards the median 

 line (PL XXVII. fig. 10). Leuckart furthermore alludes to a division of each 

 lateral muscle-mass into a dorsal and a ventral portion, but this also was not 

 to be noticed in my specimens. 



Towards the tail (PI. XXVII. fig. 13) the muscular system gradually becomes 

 less and less strongly developed, until at distances of less than 0*5 mm. clearly 

 defined fibres are not to be found. The longitudinal and oblique systems 

 appear to arise almost side by side, and the former is seen in sections a little 

 less than 1 mm. from the hinder extremity to consist of about nine bundles on 

 each side, which at this point show much less variety in size and shape than in 

 other parts of the animal. Even here, however, there are a certain number of 

 bundles, somewhat broader and more flattened, connected with the ventral 

 margin of the oblique layer of fibres. The number of fibres in each bundle varies 

 from about 10-30, but I could not make out satisfactorily any constant relation 

 between the size of the bundles and their position. 



The fibres of which the longitudinal muscles are composed are almost 

 0'008 mm. in diameter, usually oval in transverse section, except where they 

 are rendered polygonal by mutual pressure. In most cases the inner portion 

 of the fibre is pale in colour, and bordered by a fine deeply stained line 



* Loc. tit., p. 41. 



