124 M. stkaus-durckheim's 



muscles do not always arise in different species from the same 

 part. 



From these facts it may be inferred, that although the tegu- 

 mentary and muscular systems are mutually dependent one 

 upon the other, there may exist a considerable difference 

 between the modifications which these systems undergo as 

 compared with one another. This difference is often very 

 considerable ; for it is hardly possible to recognize the analo- 

 gous muscles in two species taken even from neighbouring 

 families, unless we trace them through the greater proportion 

 of the intermediate genera ; and as no species can be looked 

 upon as a type for the whole division, it is impossible to refer 

 the muscles of one species to their analogues in another, taken 

 from a different division. 



Notwithstanding these variations, it is possible, by avoiding 

 details, to lay down a few general rules. 



In the Annelida, Mijriapoda, and the larvce of insects, we 

 mostly find two principal orders of muscles, forming, the one, a 

 double series along the upper, the other, a like series along 

 the lower, part of the segments, passing from one of these to 

 another. We find these same series more or less modified in 

 the perfect insect : the lower series have become the muscles 

 which move the labium, the depressors of the head, the 

 retractors of the jugular pieces, the inferior retractors of the 

 prothorax, the praetractorsof the posterior episternal apophysis, 

 the inferior prastractors of the segments of the abdomen ; but 

 those muscles which move the head, and three thoracic seg- 

 ments, are changed considerably as to form, volume, and 

 disposition, whilst those which move the segments of the 

 abdomen, disappear whenever these segments become fixed. 

 In like manner the longitudinal dorsal series form the elevators 

 of the labrum and head, the superior retractors o^Xhe jirothorax, 

 the retractors of the wings and scutellum, the depressors and 

 prastractors of the wings, and the superior prastractors of the 

 abdominal segments. The upper part of the segment to which 

 the jugulars belong having disappeared, the muscles, which 

 would otherwise be inserted therein, proceed direct from the 

 scutellum to the head, forming the second heads of its elevators. 

 We find also in perfect insects some of those muscles, which, 

 in the larvae of insects as well as in the Scolopendrce, pass 

 from the tcrgum of the prothorax to the sternum; but, in 



