L. E. Robinson and J. Davidson 
385 
deeply staining elements of the contractile substance are the sarcostyles. 
The whole arrangement somewhat resembles the structure of the 
thoracic muscle fibres of some insects. The nuclei of the muscle fibre, 
as already mentioned, are situated in the superficial sarcoplasm imme¬ 
diately beneath the sarcolemma. They are generally much elongated in 
the longitudinal direction of the fibre, are relatively poor in chromatin 
and each contains a single nucleolus. So far as we have observed, 
the sarcoplasm of normal muscles shows no cellular division such as 
Nordenskiold has described in Ixodes ricinus, nor does the sarcoplasm 
attain any degree of thickness on the surface of the contractile sub¬ 
stance. In certain phases of development, however, we have seen 
appearances which resemble to a certain degree those observed by 
Nordenskiold in the normal body-muscle fibres of Ixodes 1 , but these 
appearances were, without doubt, due to histiogenetic phenomena. 
They were confined to the regenerating muscular tissue which is always 
in evidence in the phase accompanying a moulting period. 
This difference between Argas and Ixodes is explained by the fact 
that after engorgement, the body-muscles of the former do not undergo 
any material increase in length, while in the latter, the distention of 
the bod} 7 is relatively so enormous, that unless special provision were 
made for the elongation of the dorso-ventral body-muscles, these would 
rupture under the strain. The slow rate of engorgement of the Ixodid 
ticks allows ample time for the elongation of the muscle fibres either 
by actual growth, or, as Nordenskiold suggests, by extension of the 
sarcoplasm cells 2 . 
Almost at the points of attachment of the dorso-ventral body-muscles, 
at either extremity, to the chitinous cuticle, the contractile substance 
terminates, and the sarcolemma sheath runs out into a number of slender 
tapering tendinous fibrils which penetrate between the hypodermal cells 
and enter the substance of the chitinous cuticle, into which they pass 
for some distance and ultimately disappear. In the case of the muscles 
of the appendages, the basal attachments to the body cuticle appear to 
be identical with those of the dorso-ventral body-muscles, but at the 
other extremity, each muscle is continued for a longer or shorter 
distance as a single tendon. 
The integumental discs, as mentioned in Part I of this work (pp. 29- 
30), mark externally the sites of attachment of the muscles, both the 
dorso-ventral body-muscles and, as will be seen later, certain of the 
muscles of the appendages, which originate from the general integument 
1 Nordenskiold, E. (1908), p. 669. 2 Nordenskiold, E. (1908), p. 670. 
