360 SYNOPTIC COLLECTION. 



the female (No. 885, c, d) a broad rounded abdomen (c). 

 The arms in this sex are similar; both are small with 

 small claws (c, d). The males (No. 885, a, b, e, f), how- 

 ever, have one arm much larger than the other, while the 

 claw is greatly developed. This arm may be on the right 

 side (b) or on the left (a, e, f). It is carried across the 

 front of the body in a somewhat similar position to that of 

 the arm of a tiddler, hence the name of fiddler crab. 

 The movable jaw of the big claw in (f) has apparently 

 been broken off and another piece has grown out and 

 beyond the immovable jaw. These crabs are fighters and 

 often an arm is lost in the .fray. They walk and run side- 

 ways, but they spend much of their time in burrows which 

 they make by removing the sand and carrying it out of 

 the opening with the three anterior legs on the rear side, 

 while they climb out of the burrow by the legs of the side 

 in front. This front side may be either the right or the 

 left side of the crab, but in the male it is usually the side 

 with the big claw. 



Gonoplax rhomboides Desm. (No. 886), like Gelasimus 

 has a four-sided cephalothorax with the greatest breadth 

 in front. The stalked eyes extend out laterally nearly to 

 the edge of the carapace. The arms are long, slender 

 organs, as is the case with some burrowers, Gonoplax 

 having the habit of excavating burrows in the hardened 

 clay which are open at either end. 



It has been shown that the Crustacea offer numerous 

 and remarkable examples of adaptation of structure to 

 habit. They are also instructive in showing how a swim- 

 ming type of animal may be converted into a walking 

 type. In this process the law of cephalization or head- 

 development operates, and the organs, especially the 

 nerves and ganglia, are concentrated in the anterior part 

 of the body. They possess many characters in common 

 with the next group, the Arachnozoa. 



