100 E. A. Andrews — Anatomy of the ^nder Crab. 



calcified in the exposed or true <lorsal surface and less eo on the pro- 

 tected or ventral surface. 



In the male (fig. 2) the abdomen is narrow-triangular, and habit- 

 ually lies in a correspondingly narrow groove on the ventral face of 

 the thorax. The ventral surface is more membranous than in the 

 female. The male abdomen is also peculiar in being held in position 

 in the groove by means of a pair of small cavities (o) in the outer 

 edges of the sixth segment, which fit upon a pair of pivot-like ])ro- 

 cesses on the sterum of the second thoracic segment bearing ambu- 

 latory legs. The triangular terminal segment (a-a'i), however, which 

 bears the anal opening, is free to move downward without the move- 

 ment of the rest of the abdomen. 



In the female (fig. 3), the abdomen is broad-ovate, and when 

 not in use for carrying the eggs lies closely against the concave ven- 

 ventral surface of the thorax, almost completely covering the area 

 between the ambulatory legs. 



The abdomen may be raised till at right angles with the body. 

 The muscles moving the segments of the abdomen are small and lie 

 close to the upper and lower walls. The extensors on the dorsal sur- 

 face extend from processes on the anterior edge of each segment to 

 slight ridges in the preceding segment. Between each two succes- 

 sive segments there are two muscles, one on each side of the median 

 line. The segments are connected by a flexible membrane, and also 

 furnished with an articular process between each extensor and the 

 outer edge of the segment. On the vcnti'al surface the flexors of the 

 abdomen are similarly arranged, joining chitinons ridges on the suc- 

 cessive segments. The flexors of the first segment extend from the 

 calcified ventral ridge to the integument of the thorax. 



In front of the horizontal abdomen the ventral surface of the body 

 rises at an obtuse angle and, becoming again horizontal, terminates 

 in the rostrum. On this inclined surface a quadrangular depression^ 

 the pei-istomc, bounded on either side by the pterygostomial plates or 

 quadrant-shaped reflexed portions of the carapace is filled by the 

 mouth appendages. On the sternal plastron (the sternal surface of 

 the cephalothorax behind the peristome), four pairs of transverse 

 grooves pass towards the center and indicate the boundaries of the 

 five segments bearing the ambulatory legs. In the male a deep 

 median groove separates the two halves of the sternal surface of the 

 last of these segments. The inclined walls of the sides of the body 

 forming the floors of the branchial cavity are also marked with faint 

 corresponding lines or grooves. All these grooves are formed by 



