104 Part III. — T^oenty'Second Annual R&port 



edge of the epimeron. The truncated membranous tip of the first abdo- 

 minal segment is attached round its sides to the inside of the carapace. 

 The inner chevron is attached by a long jointed rod (r., figs. 52 and 65) to a 

 delicate muscle inserted on both sides of the bottom of the thoracic cavity. 

 A small muscle arises on the under surface of the outer chevron and joins 

 this bony rod. The outer chevron is fastened to the posterior edge of the 

 thoracic cavity by means of a membrane attached to its anterior edge. 



The muscular system of the abdomen of the Brachyura has been briefly 

 described by Duvernoy. Fig. 52 shows a median longitudinal section of 

 the abdomen of the male Cancer. Half of the abdominal muscles only 

 are of course shown. The muscles consist of flexors and extensors. 

 There are two very long flexors, arising on the thorax, and being 

 inserted one into the telson, the other into the skin covering the ventral 

 surface of the united third, fourth, and fifth joints. The abdomen is flexed 

 or extended as a single structure. The telson has movement independent of 

 the remainder of the abdomen ; thus it may, Avhen the abdomen is flexed 

 on to the thorax, be bent backwards from the thorax to permit the escape 

 of the fpeces, while the abdomen itself remains fast. Between the second 

 and third joints and between the sixth joint and telson there are pairs of 

 muscles (a flexor and extensor on each pair). At each of these joints 

 there is a larger movement than at the other abdominal joints. Between 

 1 and 2, and between 2 and 3 the action of the joint is extension and flexion : 

 the latter joint is freer than the former and afi'ords more extension than 

 any of the joints : between 5 and 6 there is flexion alone practically : and 

 between the sixth and the telson there is flexion mainly, but also extension. 



First Penis. — In the first penis there is a muscle which, arising on the 

 surface of the basal bone {h.) and also from the side of the tubular part, is 

 inserted farther up the same, vni., fig. 59, and 1, fig. 48. The muscle will 

 have the effect of tending to cause the bending of the two parts of the 

 penis towards one another. There are in addition two muscles, 2 and 3, 

 fig. 48, which arise from the outer half of the chevron and are inserted 

 into the basal bone. The upper muscle draws the first penis forward : 

 the lower tends to rotate the penis. 



The two sides of the double chevron are connected by membrane. 

 The chevrons, although fixed to the first abdominal segment, are not 

 absolutely ritfid. They are elastic. 



Sbcond Penis. — Just as in the first penis, there is also in the second 

 penis a muscle connecting the terminal part with the basal joint. In 

 this case the muscle, mu., arises on the tooth-like basal bone, figs. 61 and 

 66, and is inserted a little way up the rod. Another muscle {mu.', ib.) is 

 inserted into the same basal bone : it arises from the side of the fixed 

 arm of the second penis. A third muscle arises from the downward-bent 

 end of the arm and is inserted into a bony button-like prominence on the 

 ventral skin of the third joint (m.'", fig. 46). A long muscle arising from 

 the front of the chevron is inserted into the third joint (m.', ih.) ; and a 

 broad muscle, m.", that rises from the base of the fixed arm is inserted on 

 the anterior border of the inner cnevion. 



The Action of the Penis. 



If the genital papilla of a hard male crab is pressed spermatophores 

 may be extruded. When the abdomen of a male crab is examined the 

 genital papilla is sometimes found inserted into the tube of the first penis, 

 but oftener it is lying on the posterior surface of the base of that organ. 

 But if the first penis is drawn backwards into the position it occupies 

 when in the vagina of the female the papilla usually slips into the tube, 

 and if the fifth pereiopod is brought forward in such a way that the 



