7. CRUSTACEA. 



409 



and thus serve as a gill (figs. 411, 451). Besides, the whole body 

 surface may be respiratory and in small forms may entirely replace 

 that of the gills, so that these organs become rudimentary or may 

 entirely disappear, there being a diffuse respiration with cor- 

 responding results in the circulatory system. With a localized 

 respiration heart, arteries, capillaries, and veins are well developed, 

 but with the diffuse respiration only the heart persists as a reduced 

 structure, or with its disappearance the last traces of a circulatory 

 system are lost. 



Locomotion as well as respiration is related to the aquatic life, 

 and these animals usually possess a special form of appendage of 

 the biramous or schizopodal type, which at once differentiates these 

 forms from other arthropods. While in the latter, as every insect 

 shows, the joints of the limb follow in a single sequence, the crus- 

 tacean appendage has a two-jointed base (basiopodite), followed 



FIG. 410. Copepod appendages. I-IV, Diaptomus castor; 7, a pair of schizopodal feet; 

 //, second right antenna; 7//, right mandible; IV, right maxilla; F, right mandi- 

 ble of Cyclops coronatus. I, #, joints of basiopodite; t, endopodite; a, exopodite. 



by two many-jointed branches (fig. 410, 7), an inner or endopodite 

 and an outer or exopodite. 



The schizopodal appendage occurs only when the limb is used for 

 swimming; when it is used for walking upon the bottom, as in crayfish and 

 crabs, the exopodite is lost and only the endopodite persists as the func- 

 tional limb, which then closely resembles the appendages in the so-called 

 tracheates. This loss rarely occurs on all the appendages; usually the 

 abdominal feet and the mouth-parts retain the two-branched condition. 

 Embryology further shows that even in the case of the crabs all the feet are 

 at first schizopodal and that the walking legs lose the exopodite during 

 growth. There is some evidence to show that the schizopodal foot is not 

 the primitive type. This is furnished by the phyllopod foot (fig. 411, 77), 



