140 MUSCULAR SYSTEM. 



on the median plain by the dorsal vessel above, and the 

 nervous columns below ; and here, as in the vertebrated 

 classes, we observe that these symmetrical and voluntary 

 muscles do not cross the median plain of the body. They 

 are connected through the medium of the skin to the solid 

 coverings of the segments ; so that the exuviable skeleton 

 is cast off in all these entomoid classes without affecting the 

 insertions of the muscles into the true skin. In the air- 

 breathing, pulmonated and tracheated arachnida we find the 

 same dense, irritable, and serrated character of the muscular 

 fibres as in the perfect insects, a similar structure of the 

 muscles and their tendons, and the same general disposition 

 of these organs where the articulations resemble. Their 

 capaceous cephalo-thorax is occupied chiefly with the large 

 and strong muscles of the haunches of their numerous long 

 legs, and with those of their masticating and poisonous appa- 

 ratus. The long cylindrical segments of the post-abdomen 

 of the scorpions are chiefly occupied with the powerful 

 muscles appropriated to the movement of the poisonous sting 

 formed by the last segment of the trunk. The tendons of 

 the muscles here, as in insects and Crustacea, are only calcified 

 prolongations from the exterior skeleton. The muscular 

 system of the Crustacea forms a larger proportion of their 

 body than in the air-breathing articulata, and their move- 

 ments are more rapid and powerful than in almost any other 

 branchiated invertebrata ; their individual fibres, however, 

 are softer, more white and pellucid, less compact in their 

 texture, and less irritable and strong, than in the pulmo- 

 nated or tracheated classes ; and they require less muscular 

 force on account of the support which they receive from the 

 density of the element through which they move. The 

 muscular apparatus of their external organs of mastication 

 and of locomotion closely resembles that of insects, but their 

 stomach is covered with powerful muscles for the movement 

 of the internal teeth with which it is provided ; the ventral 

 portion of their post-abdomen is furnished with numerous 

 and strong muscles for the movements of that part of the 

 trunk in swimming, or for its retraction when alarmed ; and 

 the muscles of the prehensile terminal parts of their legs, 

 especially of the anterior pair, are proportionally large and 

 strong for the contests in which they are continually en- 

 gaged. 



