178 



PHYSIOLOGY 



The joints in these legs are all hinge joints, but there 

 are four or five in each leg and they are so arranged 

 with reference to each other that the leg has the power 

 of turning to the mouth. A rotary motion is thus pos- 

 sible comparable to that given by a single ball and 

 socket joint to the human arm. These walking legs help 

 to catch food and carry it to the mouth, where the heavy 



jaws crush it (Fig. 99). 

 These animals are great scav- 

 engers, they eat organic 

 material that contains the 

 five food substances. 



External Characteristics. 

 — The mouth at the ante- 

 rior end of the body on the 

 ventral side, the anus in the 

 posterior end, and the repro- 

 ductive openings on the base 

 of the third or fifth pair of 

 walking legs, are easily dis- 

 tinguished. The sense or- 

 gans are very prominent, 

 long antennae, and antennules, 

 very sensitive to touch, and 

 curious eyes elevated on 

 stalks. On the base of the 

 antennules are small open- 

 ings that lead into organs 

 sensitive to vibration. These are little sacs lined with 

 sensory cells each of which bears a hair. Upon these 

 hairs is balanced a tinv mass of calcium carbonate whose 



Fia. 99.— Appendages of the cray- 

 fish. 1, first antenna ; 2, mandible ; 

 3,4, first and second maxillae; 5, 

 6,7, maxillipeds ; 8, walking leg ; 

 9, pleopod. (From Hertwig.) 



