HISTOLOGY OF THE LAND-PL ARABIANS OF CEYLON. 
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plana tremellaris, Plate XIV. fig. 1, E. C. M. In Bipalium and Bhynchodemus, as will be 
seen from Plate X. fig. 5, the arrangement of the muscular fibres is extremely complex ; 
and although a general distinction may be drawn between fibres which take a circular 
course and those wdiich take a radial one, and are prolonged outwards to form the zone 
of radial fibres, yet the two sets of fibres run into one another, and no sharp line of 
distinction can be drawn between them. In Planarians with a less highly specialized 
muscular system, such as Dendrocoelum or Leptoplana , the case is different, and here the 
muscles are sharply divided into systems. The radial fibres of Bipalium and Bhynclio- 
demus appear to correspond with the vertical or dorso-ventral fibres of these aquatic 
species, whilst the irregularly disposed circular fibres are homologous with the definite 
circular layer of the same animals. 
Radial fibres pass outwards in Bipalium and Bhynchodemus in every direction, to form 
the clear zone already described as existing beneath the skin-muscles. Specially stout 
muscular fibres, derived from the circular system, pass transversely immediately beneath 
the digestive tract (Plate X. figs. 5 & 6), and a series of transverse fibres in the region 
of the ambulacral line is to be found about the level of the inferior boundaries of the 
main water- vascular canals (Plate X. fig. 5). Stout vertical fibres form the lateral walls 
of the central digestive tube, and mingled with these are finer fibres which bend over to 
form a sort of circular muscular layer to the intestinal tube. The septa between the 
diverticula of the intestine are formed by very fine fibres, which are continuous with 
fibres belonging to the body-mass, and which may be seen in Plate X. fig. 5 (where 
portions of the septa remain in situ) to have a decussating arrangement. A similar 
arrangement may be seen in Bhynchodemus (fig. 7). Max Schultze (loc. cit. 4) remarks 
on a difference between the special muscular fibres of the intestine in Geoplana and the 
other motor body-muscles; and the extreme fineness of. the muscular fibres of the intes- 
tinal septa above mentioned points to a similar histological differentiation in Bipalium 
and Bhynchodemus. 
When viewed in longitudinal sections, the circular muscular fibres of the body-mass 
are seen to take the same oblique direction with regard to the long axis of the body 
which is to be observed in the external circular muscles. The fibres decussate in a 
similar manner, and take then a more or less spiral course. In the head, bundles of 
fibres may be seen crossing one another along the middle line, and spreading out right 
and left towards the margins of that curious semilunar structure. 
Ambulacral line. — In Bipalium, just before the contour of the under surface of the 
body begins to swell out to form the projecting ambulacral line, the external circular 
muscular layer splits into two parts (Plate XI. fig. 3) — one external, which, preserving 
the same direction which it takes in the other regions of the body, forms a thin layer 
immediately beneath the basement membrane of the skin; the other passes directly 
inwards, towards the median muscular mass of the ambulacral line, and its fibres then 
separating spread out fanwise and become lost in this mass. In so doing it separates 
off a series of smaller longitudinal muscular bands from the larger one, forming the 
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