194 HIGH SCHOOL ZOOLOGY. 



racic legs — the great claws — are adapted for prehension ; they, 

 like the fifth and sixth, are chelate, i.e., the penultimate joint 

 is prolonged so as to be opposed to the terminal joint. 



Of the five pairs of head-appendages, the two anterior 

 (antennulce and antenTice) are sensory, while the three poste- 

 nor (the mandibles and the maxillce) are related to the mouth- 

 aperture as jaws. The second pair of maxillae most closely re- 

 semble the foot-jaws, but the exopodite and epipodite of each 

 are united into a spoon-shaped flap, which lies in the antei-ior 

 narrow apei-ture of the gill-cavity, and, by its movements, cre- 

 ates a current of water, which flows outward through that aper- 

 ture. In both pairs of maxillsB as well as in the mandibles, the 

 endopodites are feeler-like (palps), while it is the basal segments 

 which are flattened and approximated to the mouth-aperture, 

 those of the mandibles alone being hardened for cutting. Neither 

 exopodites nor epipodites are present in the mandibles or first 

 pair of maxillse. On the other hand, both of the foremost ap- 

 pendages are biramous, the exoi)odites of the antennae being, 

 however, mere scales, while those of the antennulse are similar 

 to the endopodites. On the basal joints of the antennae and 

 antennulae, respectively, are to be seen the apertures of the 

 green glands or kidneys and of the eara to be afterwards de- 

 scribed. 



6. Having inspected the outward form of the body, we must 

 now glance at the various systems of organs. It will be ob- 

 served that the chitinous cuticle remains soft where movements 

 are necessary, and that it is most densely calcified where it 

 meets with the greatest strain, as e.g., in the chelae and 

 mandibles. 



7. The muscles are formed of very plainly striped tissue ; 

 indeed the histology of this tissue can be more easily studied 

 here than in the catfish (I, 8). The muscular bundles are at- 

 tached to ingrowths of the exoskeleton, which can be seen very 



