72 Mr. H. M. Bernard on the Apodemes of Apus 



We liave now, lastly, to explain the changes which have 

 taken place in the ventral longitudinal bands of Apus and 

 their attachments, to form the flexor muscles of Astacus with 

 their somewhat different attachments. 



In Apus the muscle-bands are attached solely to the 

 endopleurites. Anteriorly they are attached to the large 

 apodeme behind the underlip, and run backward in a band 

 until they gradually widen out to form, in the limbless 

 segments, a dermo-muscular tube. 



In Astacus a great change has taken place. As above 

 described, the anterior part of the thorax has undergone 

 lateral as well as longitudinal compression, due to the trans- 

 formation of legs into maxillipedes. The lateral compression 

 caused the longitudinal bands of the two sides to meet in 

 the middle line, with a consequent fusing of the sinewy 

 segmental partitions of the one band with those of the other. 

 The longitudinal compression caused the ventral segmental 

 constrictions to become the fixed permanent endosternites, 

 which were pressed up till their inner edges fused with these 

 same sinewy partitions. 



When it was no longer necessary further to compress 

 the thorax, in order to turn the anterior trunk-limbs into 

 maxillipedes, the greater part of the muscles degenerated, 

 leaving, however, the branches of the endopleurites bound to 

 the endosternites by the sinewy tissue which persisted after 

 the muscle elements disappeared. The most important parts 

 of the ventral longitudinal bands which were retained were 

 those which ran either downward into the limbs or back- 

 ward into the abdomen. Of the former, we find the coxal 

 muscles in Astacus attached to the sinewy capitals of the 

 pillar-like endosternites. We have, in fact, muscles pulling 

 outwards attached to each side of a fold of the skin ! It is 

 evident that this must have been a secondary arrangement, 

 as no fold of the skin could possibly have ai'isen under these 

 circumstances. There is, however, no difficulty if we refer 

 to Apus. There we find these coxal muscles springing from 

 the sinewy partitions in the ventral muscle-bands. By the 

 longitudinal compression of the thorax already mentioned, the 

 segmental constrictions in Astacus were forced up until their 

 inner edges fused with these sinewy partitions. Hence the 

 coxal muscles naturally come to descend from the upper 

 edges of the segmental constrictions or capitals of the endo- 

 sternites. 



The abdominal muscles require little notice. The order of 

 their attachments to the endosternites of the thorax is jusc 

 what we should have expected from their origin. As we go 



