NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 79 



tergal and sternal plates, and the primary intersegmental grooves 

 become the submarginal antecostal sutures. As a consequence, a new, 

 secondary type of segmentation has been established, in which the 

 functional intersegmental rings are the membranous posterior parts 

 of the primary segments, and the action of the longitudinal muscles 

 becomes intersegmental instead of intrasegmental. A body mechanism 

 of this kind is typical of all the arthropods, but still it is by no means 

 fixed, for innumerable modifications of it have been introduced 

 in adaptation to the development of special structures for specific 

 purposes. 



The acquisition of an exoskeleton necessarily limits freedom of body 

 movement, such as that possessed by the highly flexible annelids, 

 but at the same time it furnishes a mechanism by which movements 

 may become more specific, since the development of definite hinge 

 joints becomes possible, and muscles can assume more effective 

 antagonistic relations to each other. The longitudinal muscles lose 

 nothing of their efficiency, but their contraction now results in a 

 telescoping of the body segments. The presence of dorsal and ventral 

 plates, however, necessarily eliminates the constrictor effect of the 

 primitive circular or semicircular muscles ; the latter, therefore, have 

 become reduced to lateral tergosternal muscles, the contraction of 

 which produces a flattening of the body. The primitive mechanism 

 of dilation and extension by unequal distribution of internal pressure 

 is still operative ; but the potentiality of developing endoskeletal 

 structures gives the possibility of a new mechanism of expansion, for 

 the ingrowth of apodemal arms from tergal or sternal areas, on which 

 primarily compressor muscles are attached, may reverse the position 

 of such muscles to the extent that they become dilators. A separation 

 of contiguous plates, however, may be brought about also by the 

 contraction of intersegmental muscles that have been reversed by 

 the overlapping of the plates. All these mechanical devices and many 

 others are variously and often highly developed in the different 

 arthropod groups, and their elaboration has set the arthropods far 

 above the annelids and onychophorons in the power of performing 

 definite and specific acts. Even the wing mechanism of pterygote 

 insects has been built up from little more than the skeletal parts and 

 musculature common to the body segments. It should be observed, 

 however, that although the musculature of the body segments and 

 the appendages is fairly definite and fixed within the major arthropod 

 groups, there seems to be no limit to the potential genesis of new 

 muscles in connection with special organs, such as the male genitalia 

 of insects, and, furthermore, that the entire body musculature is 



