NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN 35 



The second, third, and fourth pereiopods resemhle one another 

 rather closely, as they are nearly the same in size and perform the same 

 kinds of motions, being adapted for walking. In these, the dactylopo- 

 dite arises on the distal part of the propodite, tapering rapidly and 

 becoming much flattened. The tip is pointed and sharp, and on these 

 tips the crab is able to walk. The overhang of the carapace allows little 

 upward motion to these legs, and so they have retained the normal posi- 

 tion of hanging downward beneath the body. The anterior surface of 

 these legs is preaxial, as is usually the case in arthropods. 



The fifth and last pereiopod is the swimming leg, and projects back- 

 ward and upward behind the carapace when the crab is swimming. Its 

 basal muscles are very powerful, especially the remotor. which is rela- 

 tively weak in the preceding pereiopods. The terminal segment is very 

 thin and flat like the blade of a paddle, ovoid in shape, and propels the 

 crab sidewise very swiftly. Like the first pereiopod, the fifth is also a 

 half -turn away from its normal position, but in a direction opposite to 

 that of the first, so that its anterior (preaxial) face is now downward, 

 and its postaxial face uppermost. 



Since the muscles of the segments distal to the basipodite are essen- 

 tially similar in all the pereiopods, those of the third pereiopod have 

 been chosen to be described in detail, while the corresponding muscles 

 of the other legs may be referred to the third as a model, taking into 

 consideration the fact that the first and fifth legs are not identical with 

 it in position. The basal muscles are sufficiently different in each leg 

 to merit a full description. 



A cross-section of the body at the level of the anterior part of the 

 fourth and of the sixth thoracic segments shows the relations of some 

 of the muscles of the first and third legs to their respective surround- 

 ings. (See fig. 13.) 



The promotor of the fifth pereiopod deserves notice because of the 

 peculiar disposition of its anterior branch. This projects forward 

 through the thorax into the fourth thoracic segment, surrounded by a 

 membrane, on the posterior surface of which its own fibers originate, 

 and on the anterior surface of which about a dozen branches of muscles 

 pertaining to the legs of the fourth, fifth, sixth, and seventh segments 

 also take their origin. 



Another feature of the endoskeletal structure must here be ex- 

 plained. An intermediate endopleurite exists in the center of each of 

 the basal chambers occupied by the fourth, fifth, sixth, and seventh seg- 

 ments. This endopleurite is fastened to the membrane covering the 



