708 A. T. EVANS 
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Fig. 2 Cross-section of main trunk of frontal appendage of Thamnocephalus 
platyurus; A, mid-dorsal mass of modified columnar epithelium; B, lateral un- 
modified epithelium; C, area of areolar, adipose, and connective tissues; D, 
lateral area of modified columnar epithelium, forming lateral insertion for flexor 
muscle; H, flexor muscle; F, ventral sinus; G, ventral mass of modified columnar 
epithelium; H, mesial junction of the dorsal and ventral masses of modified 
columnar epithelium; the dissepiment thus formed is the inner insertion of both 
flexor muscles; 7, chitinous covering of the appendage. 
quite similar, each containing in its dorsal portion a considerable 
amount of adipose and connective tissue; and in its ventral por- 
tion a large flexor muscle and a conspicuous crescentic sinus. 
This sinus is located just inside of the ventral layer of epithe- 
lium, in the angle formed by the attachment of the inner edge of 
the flexor muscle to the central dissepiment of modified columnar 
epithelium. 
The columnar epithelial cells occupying the mid-dorsal and 
mid-ventral regions and the extreme lateral portions of the 
cross-section—that is, the points of attachment of the flexor 
muscles—are of a modified type. They contain large numbers 
of very minute fibrils in the cytoplasm, which give them strength 
for the attachment of muscles. This particular form of modified 
