MUSCLES OF AETHEOPODA. 251 



Muscular System. 

 § 194. 



The muscular system of tlie Arthropoda is not homogeneous in 

 character, like the circular or longitudinal layers in the dermo- 

 muscular tube of the Vermes. It is more differentiated, and we 

 find separate bundles composed of a varying number of trans- 

 versely-striped muscular fibres. Peripatus is the only exception to 

 this, and its muscular system, owing to the absence of transverse 

 stria? from the muscular elements, rather resembles that of the 

 Vermes. The dermo-muscular tube is however generally converted 

 into a complex of separate muscles, which together form a muscular 

 system. As the skeleton of the Arthropoda is an external one, 

 the muscles arise from and are inserted into the inner face of the 

 hollow cylinder, or portions of a cylinder, which are formed by the 

 segments of the body as well as by those of the appendages. This 

 development of a dermal skeleton is to be regarded also as an 

 important factor in the differentiation of the muscular system, in so 

 far as separate muscles can only be formed when they have a firm 

 point of origin and of insertion. In the number as well as in the 

 varied arrangement of the separate muscles, the muscular system 

 presents a high grade of development, which always corresponds to 

 the varying significance and development of the metameres. It 

 differs from the muscular system of the Annelides in corre- 

 spondence with the difference expressed by the homonomy of 

 their metameres, as compared with the heteronomous ones of the 

 Arthropoda. 



When the metameres are similar the muscles of the metameres 

 are also similar, and when the separate metameres are unequally 

 developed, either by the fusion of a small, or a larger number into 

 larger divisions of the body, or by atrophy, there is a corresponding 

 want of similarity in the arrangement of the muscles of those parts. 

 The development of the appendages greatly influences the develop- 

 ment of the muscular system, and when the metameres which carry 

 appendages are greatly enlarged in comparison with the rest, the 

 muscular system takes a large share in the increase. 



The numerical relation and arrangement of the muscles often 

 undergo considerable alterations in those Arthropoda that undergo 

 metamorphosis. This applies as much to progressive as to retro- 

 grade metamorphosis. In the former the change leads to a differ- 

 entiation into unequal groups ; in the latter to an atrophy of very 

 large parts of the system ; this obtains in the parasitic Crustacea 

 and in the fixed forms. 



