330 MR. E. W. SHANN ON THE 
to below posteriorly (like those of the superficial area of the 
mesio-dorsal portion). 
It is this change in direction of the fibres which led Humphry 
and Maurer to distinguish two layers in the ventral moiety of 
the lateral muscle. If Chevrel’s account of the internal structure 
of a myomere has been followed, the reason for this change in 
direction of the fibres will be perfectly clear. Since the ventral 
half of the dorsal moiety of the myomere takes the form of a 
pyramid with its apex directed anteriorly, of the fibres running 
from the apex the uppermost ones may be expected to take a 
direction from below anteriorly to above posteriorly, and the 
lower ones from above anteriorly to below posteriorly ; and this 
is indeed the case. Moreover, since the apex is directed inwards 
and downwards, only the outer upper faces of the pyramids on 
which the fibres are directed from below anteriorly to above 
posteriorly, is visible externally, and has been designated the 
“latero-dorsal portion” in our superficial examination. The 
lower face, on the other hand, in which the fibres are directed 
from above anteriorly to below posteriorly, is hidden beneath the 
surface. 
Similarly, the portion which has been designated “ mesio- 
dorsal” in our superficial examination represents the outer face 
of a backwardly directed pyramid. 
It has already been mentioned that, just behind the head, near 
the insertions of the mesio-dorsal and latero-dorsal portions of 
the lateral muscle, these two masses are divided. The division 
is not merely superficial, but extends usually throughout their 
depth ; though in some forms (Cotius scorpius) a certain number 
of strands from their deeper contiguous faces serve to connect 
them almost up to the point of their insertions. 
The above description of the arrangement of the dorsal moiety 
of a myomere of the caudal region applies equally well to the 
ventral moiety in the same region, only the direction of the 
apices of the pyramids is reversed, the dorsal one being directed 
anteriorly, the ventral posteriorly. Here, again, only the outer 
face, but in this case the lower, of the anteriorly directed 
pyramid, with its fibres running from above anteriorly to below 
posteriorly, is visible externally, where it is recognised as the 
“Jatero-ventral portion” of our superficial examination; while 
beneath it lies the upper face, in which the fibres run in the 
opposite direction, namely, from below anteriorly to above 
posteriorly. Hence the explanation of the description in this 
portion of the lateral muscle of two superimposed strata, equi- 
valent respectively to the obliquus externus and obliquus internus 
of higher vertebrates, advanced by Humphry and Maurer. 
In the abdominal region, where the walls of the ventral moiety 
are of necessity thin, the conical structure of the myomere 
is lost. The latero-ventral portion throughout its depth con- 
tains fibres which are directed from above anteriorly to below 
posteriorly, at least in its most anterior area. It acquires its 
