/. CRUSTACEA. 



409 



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

 surface may he respiratory and iu 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 

 res2)iration heurt, 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 arpiatic life, 

 and these ajiimals usually possess a special form of appendage of 

 the biramous or srldzopodal 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. — ClMpepiKl appenflaKe.s. 1-lV, Diaplomus ca.^ii})-: 7, a pair of schizopodal feet; 



;/, seciind I- 

 >le uf Clfcloi' 



:ht antenna; i//, I'lL^ht mandible; /r, rii^ht itia.xilla; T\ rii^lit niandi- 

 titn.-i. ?, .', j(»iiits of basiojKidite; /, endupudite; ((, exopodlte. 



by two many-jointed branches (fig. 410,/), an inner or endopodile 

 and an outer or e.copodite. 



The schizopodal appendage occurs only when llie limb is used for 

 swiiiimiiig; when it is used for wallcing upon the bottom, as in craytisli and 

 craljs, the c.xopodite is lost and only the endopodite .persists as the func- 

 tion;!] limb, wliieh tlien closely resembles the appendages in the so-called 

 ti'ache;ites. Tills loss rarely occurs on all the appendages; usually the 

 abdominal feet and the mouth-p;u'ts retain the two-liranched comlition. 

 Embi'yology further shows that even in the case of tlie craljs all the feet are 

 at first schizn[)odal and thp.t the walking legs lose the e.xopodite during 

 growth. Tliere is some evidence to .show that tlie schizopodal foot is not 

 the primitive type. This is furnished by the phyllopod foot (fig. 411, //j, 



