HISTOLOGY OF THE LAND-PLAN AEI AN S OF CEYLON. 
125 
and other observers have called the external tissue of the Planarians they have 
examined a basement membrane, and have therefore described longitudinal muscles as 
being external in that animal — which no doubt is correct from one point of view, since 
it is probably impossible to detect any muscular fibres in the external tissue of the 
body ; but still it is apt to lead to an erroneous conclusion. The great fact to be borne 
in mind is that, whatever this external tunic may be called, it is the homologue of the 
external circular muscular coat of higher worms and Bipalium, and that therefore the 
distinction between the arrangements of the muscular system in the two groups is of 
very little importance. MTntosh (loc. cit. p. 310) lays great stress on the fact that in 
Ommatoplea alba the circular muscles are external and the longitudinal internal, whilst 
in Borlasia the reverse is the case ; and accordingly he regards these two worms as 
belonging to very different types indeed. Now it would be difficult to overestimate the 
wide gulf which would exist between these two forms if there were really any inversion 
of the muscular coats here ; but the external circular coat of Ommatoplea is evidently 
the homologue of the thick external tunic of Borlasia , called by MTntosii the base- 
ment membrane, since in Ommatoplea there is said to be no basement membrane, and 
the external circular muscular coat lies immediately beneath the epidermis, as does the 
so-called basement membrane of Borlasia. It will be noted that the extremely thin 
and delicate basement membrane 'which intervenes between the external circular mus- 
cular coat in Bipalium and the epidermis has nothing to do with the thick tunics (as I 
believe, improperly termed basement membranes) of Borlasia and Leptoplana. These 
latter are probably contractile, and perform the part of muscular tunics, although no 
definite fibrillar arrangement has been detected in them. Immediately beneath the 
external circular muscular layers are the longitudinal muscles of the superficial muscu- 
lar system. These muscles do not form a continuous tunic to the body as the circular 
muscles, but occur as isolated bundles of fibres. The fibres composing these bundles 
are remarkably stout, and the bundles themselves are arranged beneath the external 
circular layer at tolerably regular distances from one another, the intervals between 
them being the situation occupied by eye-spots and the fine upper extremities of the 
parent cells of the rod-like bodies (see below, p. 144 seqq.), and also by terminations of the 
radiating muscular fibres. These longitudinal fibres are well seen in section in Plate 
XI. figs. 1, 2, & 9 and Plate X. fig. 13, and as viewed from the surface of the body in 
Plate XI. fig. 5. In BTiynchodemus the fibres do not form such definite isolated bundles 
as in Bipalium , as may be seen by a comparison of figs. 1 & 2 in Plate XI. The 
muscular system generally in Bipalium is far more highly specialized than it is in 
Bhynchodemus. This longitudinal system of muscles is most fully developed in the 
middle of the dorsal surface of the body and in the infero-lateral regions, in corre- 
spondence with the greater elaboration of the external muscular coat in those regions 
already described. 
This superficial longitudinal muscular system is evidently represented in the leech by 
the few longitudinal fibres which, according to Leuckart (loc. cit. p. 645), are to be 
