128 
ME. H. N. MOSELEY ON THE ANATOMY AND 
regular external layer of longitudinal muscles. This latter layer, after being continued 
a short distance inwards, is lost. The series of smaller longitudinal muscular bands is 
continued all over the ambulacral line, as may be seen by reference to the figure referred 
to above. The stout vertical muscles of the line swell out as they pass between these 
longitudinal bundles to end at the external transverse or circular muscular layer. In 
Rhynchodemus the ambulacral line is merely marked by an absence of pigment, and a 
slight increase in strength of the vertical fibres. 
In Dendrocoelum lacteum , as may be seen from Plate XIV. fig. 7, a well-defined 
external circular muscular coat exists, to which are attached the stout bundles of vertical 
or dorso-ventral fibres. The dorso-ventral fibres pass between bundles of longitudinal 
fibres, just as do the radiating fibres of Bipalium and Rhynchodemus between the super- 
ficial longitudinal muscular bundles. The cells of the rod-like bodies lie just beneath 
and between the bundles of longitudinal fibres, in fact bear the same relation to them 
as in Bipalium and Rhynchodemus. Internally to these well-defined muscular elements, 
and between these and the digestive cavity, is an interval filled up with a pulpy tissue, 
which is not so perfectly differentiated histologically. In this tissue are embedded the 
thread-cells and connective-tissue elements, and in cavities excavated in its substance the 
generative organs and water-vascular system. It evidently corresponds to the internal 
muscular mass of Bipalium. On its outer margin can be traced fibres having a circular 
arrangement ; but I was unable to detect a second series of definite longitudinal fibres in it. 
In j Leptoplana tremellaris (Plate XIV. fig. 1) the external circular muscular coat is 
reduced to a mere membrane, and this is succeeded by a layer of longitudinal fibres. 
In their histological development the elements of the body-mass lying internally to 
this are much more perfectly differentiated than in Dendrocoelum lacteum. The 
internal circular muscular layer is well defined, and is succeeded in most regions of 
the body by a region occupied by numerous longitudinal fibres. It appears, then, on the 
whole, that the arrangement of the muscular fibres in the bodies of Planarians, and 
indeed all Turbellarians, is essentially the same as that in other worms. The external 
muscular coat is circular, the internal longitudinal, though in some cases the external 
coat becomes rudimentary, and appears as a simple membrane ; and this may occur in 
different parts of the same animal. The external longitudinal muscles are succeeded by 
internal circular muscles, distinctly marked as a separate layer in Leptoplana , and just 
to be made out as such in Dendrocoelum , interspersed between the internal longitudinal 
fibres of the body-mass in Bipalium and Rhynchodemus , but in this latter forming in 
some parts of the body a distinct layer, as in Leptoplana (see Plate X. fig. 7). The 
interval forming the zone occupied by radiating fibres in Bipalium and Rhynchodemus is 
absent in the flattened aquatic species. 
The following diagrams represent the arrangements and homologies of the various 
muscular layers in Bipalium , Rhynchodemus , Leptoplana, and Dendrocoelum (A, ex- 
ternal circular layer ; B, longitudinal ; C, D, internal circular and longitudinal muscular 
systems respectively) : — 
