NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN 5 



at their origin but diverging toward their insertion upon the outer 

 walls in the middle of the sixth segment. There is no ventral muscle 

 connecting the sixth segment with the telson in either sex. The ventral 

 superficial muscles are much heavier in the female than in the male, 

 owing no doubt to the fact that the " locking " device for the male's 

 abdomen precludes the necessity for any strong contraction toward the 

 body. The female, on the other hand, has no such locking device but 

 must hold the abdomen bent forward under the body or curled around 

 the egg mass, this position of the abdomen necessitating heavier 

 muscles. 



7 a, b. Muscidus dilatator ani (fig. i A, B). — The main part of this 

 muscle arises on a triangular cartilagelike thickening on the ventral 

 membrane lying between the posterior border of the sixth somite and 

 the anterior border of the telson. It is inserted ventro-medially by the 

 side of the anal opening. The small second part arises in the same 

 cartilagelike thickening on the ventral membrane, and is inserted on 

 the anterior dorsal wall of the telson. By the contractions of the two 

 muscles the anus is opened and widened, while the elasticity of the 

 membrane around the anus opposes them. 



8- 1 J. Musculi dorsales superficiales abdominis (fig. i A). — While 

 Astacus has its first superficial dorsal muscle connecting the thorax 

 with the abdomen, this muscle does not occur either in Pandalus or in 

 Callincctes. A very heavy U-shaped membrane connects the first ab- 

 dominal segment with the thorax in Callinectes, and at the base of this 

 membrane arises the first pair of dorsal superficial muscles (8), which 

 thus corresponds to the second pair in Astacus. Each muscle of this 

 pair is in several parts lying side by side. The next pair (p) arises near 

 the middle of the second segment behind a heavy sclerotized ridge and 

 is inserted on the anterior border of the following segment, which in 

 the male crab represents the complete fusion of the third, fourth, and 

 fifth abdominal somites. In the center of this fused section there is still, 

 strange to say, a pair of definite patches of muscle tissue arising on a 

 heavy ridge, the marks of attachment of which may be seen going 

 through to the dorsal integument as two slight shallow depressions. 

 This pair of muscles (numbered " lO or ii" in the figure) probably 

 represents either the fourth or fifth pair of dorsal superficial muscles. 

 It appears to have no function, as the hinge to its somite is entirely 

 immovable. The adjacent pair of muscles has completely disappeared 

 in the male. The sixth pair {12) arises some distance within the fused 

 segment and is inserted on a cartilagelike outgrowth from the anterior 

 border of the sixth segment. The seventh pair (ij) is long and very 



