no. 9 musculature of the blue crab cochran 53 



The General Structure of the Crustacean Appendage 



In order to understand the true relationships between the exceed- 

 ingly diverse and often highly specialized crustaceans that exist today, 

 it is a matter of importance to attempt to reconstruct a generalized 

 ancestral type, from which all these existing divergences may have 

 arisen by various evolutionary processes. 



A typical leg of any of the higher crustaceans consists of not more 

 than seven segments, including the basal segment called the coxopo- 

 dite, which is followed by the basipodite bearing the endopodite of 

 five segments, each segment having a pair of muscles to move it. Any 

 or all of these seven segments may be provided with exites — lobes 

 growing on the external part of the limb, or endites — lobes growing on 

 the internal part of the limb. These exites and endites, when they 

 are large and movable, may have special muscles of their own. 



In the insects the basal segment of the leg is obviously divided into 

 a coxa and a subcoxa, the latter forming sclerotized plates in the 

 pleural wall of the thorax. In the crustaceans it is possible to trace a 

 similar development of the limb basis. Consequently, we may look 

 upon the coxopodite as being equivalent to the coxa of the insect, 

 while the sternal and possibly the pleural regions of the thorax in the 

 blue crab represent the subcoxal regions of the legs of the insect. 



The coxopodite is sometimes ankylosed with the basipodite, in 

 which case the resulting structure goes by the name of protopodite. 

 The coxopodite may exist by itself, as in the mandible and the two 

 maxillae of the isopod and the amphipod (fig. 21 A, B, C; fig. 22 A, 

 B, C) , or it may give rise to a basipodite with or without an exopodite 

 and endopodite. The coxopodite may have one or more epipodites 

 (fig. 24 E, F), which are usually gill-like, nonsegmented structures 

 forming a part of the respiratory system. 



In the lower crustaceans the leg has an exopodite as well as an en- 

 dopodite, both of which always arise from the basipodite. In the higher 

 crustaceans the exopodite still persists in the maxillipeds and the 

 pleopods. 



The exopodite may have any number of joints, and its distal part 

 may be modified to form a flagellum, as in the maxilliped and true 

 legs of the mysid (fig. 19 D ; fig. 20 A, B, C). The endopodite, on the 

 contrary, is very definitely limited to a maximum of five segments. 

 Frequently, the distal segments are not present, and some of the 

 proximal ones may have ankylosed. The endopodite exists in its 

 typical form as a walking leg in the higher crustaceans, the names 

 of its segments being the ischiopodite, the meropodite, the carpopo- 

 dite, the propodite, and the dactylopodite. The typical crustacean leg 

 has two principal places for bending — one at the basal joint between 



