NO. 9 MUSCULATURE OF THE BLUE CRAB COCHRAN I3 



of the membrane pulls them sharply apart. Apparently there are no 

 special muscles within the flagella themselves. 



THE SECOND ANTENNA 



In the blue crab the second antenna is so different in structure from 

 the corresponding appendage in the crayfish and shrimp that it is not 

 feasible to attempt to draw a parallel very closely between them. The 

 second antenna in the crayfish, as Schmidt remarks in his masterly 

 analysis (Schmidt, 1915, p. 205), is the most highly segmented of all 

 the head appendages, and hence possesses the greatest ability for 

 motion. The same complicated structure was observed by Miss Berke- 

 ley in the shrimp Pandalus. Both these crustaceans have a well-devel- 

 oped, heavily muscled exopodite, as well as an endopodite in which all 

 the typical segments may be recognized, the flagellum being taken to 

 represent the dactylopodite in both cases. 



There is no jointed exopodite in the blue crab ; the only trace of it 

 is a hard protuberance on the outer part of the basipodite. Since a 

 complete fusion has taken place between the basipodite and the head 

 carapace, there are no depressor or levator muscles. The coxopodite 

 is reduced externally to a membranous pocket lying anteriorly between 

 the basipodite and the head carapace, in which the fusion occurs poste- 

 riorly. Arising from the basipodite, and forming the base of the endo- 

 podite, come two segments which I shall arbitrarily call the ischiopo- 

 dite and the meropodite, which are provided with the typical reductor 

 and productor muscles. Following these is a long annulated flagellum 

 without definite muscles inside it. It is impossible to say whether the 

 flagellum represents the division of the last three segments of the 

 normal endopodite — carpopodite, propodite, and dactylopodite — or of 

 the carpopodite alone, if one wishes to assume the complete loss of the 

 other two. Because of this uncertainty, the muscles lying in the so- 

 called meropodite and controlling the action of the flagellum are re- 

 ferred to as the reductor and productor of the flagellum. 



j_^. Musculus promotor II antennae (fig. 4). — This muscle arises 

 on the dorsal carapace in the protogastric region, and runs inward and 

 forward to its attachment on a slender tendonlike structure which 

 thickens and hardens into a sickel-shaped rod, which curves outward 

 and forward beneath the membranous pouch lying between the basi- 

 podite and the head carapace, and finally attaches itself to this same 

 cartilagelike membrane, which is moved forward and inward by its 

 action. 



