172 MR W. E. HOYLE ON 



(sarcolemma ?), in which is often seen an ovoid thickening which may be due 

 to the presence of a nucleus. When they are well preserved these fibres show 

 a clear transverse striation, the striae being separated from each other by a 

 distance somewhat greater than the diameter of the fibre ; thus resembling 

 those of the oblique fibres shown in PL XXVII. fig. 9. 



3. The Oblique System of muscles does not form a complete layer encircling 

 the body, but consists of two muscular planes, which take origin near the 

 ventral median line, and pass upwards and outwards, diverging at an angle of 

 sixty degrees or more (PI. XXVII. fig. 10, m.o). Each plane appears in trans- 

 verse section as a thin line, never thicker, and usually somewhat thinner, than 

 a single fibre of the longitudinal muscles, but it shows a longitudinal striation 

 as if composed of several fibrils. Near the ventral margin, and to some extent 

 also near the dorsal, this lamella is seen to split into a varying number of 

 thinner portions, between which are situated the longitudinal muscle-bundles 

 as already described. 



This muscular group, however, is oblique in two senses ; as we have just 

 seen, its fibres form two lamellae inclined towards the median plane of the 

 body, but in addition to this, they pass obliquely downwards and forwards 

 from one segment of the body into the next, as is shown in PI. XXVII. 

 fig. 17. These fibres appear to be homologous with one half of the cruciform 

 systems, which Leuckart describes and figures in P. proboscideum* but of the 

 other half, those, namely, which proceed downwards and backwards, no trace 

 was to be found. 



If it be correct to assume that the crucial fibres are the remains of one or 

 two complete coats of oblique fibres, if Ave suppose the process of degradation 

 carried still further, then the fibres passing downwards and backwards, not 

 being of so great utility in the movement of creeping as those passing in the 

 other direction, would be the first to disappear, and these latter only would 

 remain. 



The fibrils of which this layer is composed are about 0*003 mm. in diameter, 

 but they frequently exhibit a longitudinal striation, as though made up of still 

 finer elements, and rarely the transverse striation was clearly exhibited 

 (fig. 9, m.o). At intervals along the fibrils small darkly-stained nuclei are to be 

 observed, but whether these belong to the sarcolemma or to the connective 

 tissue, I was unable to discover. 



With respect to the mode of termination of the fibrils I can say but little ; 

 it was often quite easy to follow them as far as the subcuticular epidermis, and 

 in a few cases an appearance was presented such as is shown in PI. XXVI I. 

 fig 16, giving decidedly the impression that they passed between the cells of 

 this layer and are inserted directly into the cuticle. It would hardly be safe 



* 



Loc. cit, p. 41, Tab. i. fig. 10. 



