526 ZOOLOGY SECT. 



portion or stem, the protopodite ; a small inner branch, the endopodite ; 

 and a large outer branch, the exopodite. This second appendage 

 becomes the antenna of the adult, and may be called the antennary 

 foot : it is the chief organ of locomotion of the larva. The second- 

 trunk appendage is the mandibular foot (3), so called because it 

 becomes converted into the mandible of the adult : it is also 

 biramous. The only internal structure to be noted is the straight 

 enteric canal with its dilated anterior end or stomach : the mouth 

 opens between the bases of the antennary and mandibular feet, 

 and is bounded in front by a large labrum : the anus is at the 

 extremity of the anal region. This very peculiar and characteristic 

 larval form is called a nauplius. 1 



The nauplius swims freely, chiefly by vigorous strokes of the 

 great antennary feet, and after a time undergoes a series of moults 

 or ecdyses, the cuticle being cast off and the animal emerging 

 in the form shown in Fig. 436, B. The trunk-region has elongated, 

 new segments having been added, as in Cheetopods, between 

 those previously present and the anal region. The antennules 

 have become shifted backwards, and rudiments of a fourth pair 

 of appendages, the first maxillae (4), have appeared. The carapace 

 has grown out from the dorsal region of the head, and a peculiar 

 paired sense-organ (fs.) has appeared on the head. 



After two more ecdyses the larva has assumed the form shown 

 in Fig. 436, C. Several new segments have been added, and the 

 anterior of these all bear leaf-like thoracic feet. The antennary 

 feet are still very large, and the bases of the mandibular feet have 

 become enlarged and toothed so as to form biting jaws. The 

 carapace (s) has increased greatly, and the caudal styles have 

 attained a considerable size. Further moults occur, new segments 

 are added with their appendages, the antennules and antennae 

 degenerate the latter sometimes disappearing altogether, the 

 mandibles become reduced to the enlarged basal segment, and the 

 larva passes by almost insensible gradations into the adult form. 



It will be seen .that the development of Apus proves clearly 

 that the antennae and mandibles are ordinary trunk-appendages, 

 homologous with the thoracic and abdominal feet : a comparison 

 of the antennary and thoracic feet of the larva supports the view 

 that the endopodite of the former corresponds with the fifth endite 

 of the latter, and the exopodite with the sixth endite. The 

 antennules are from the first unbranched or uniramous, and are 

 originally situated quite at the anterior region of the body : they 

 do not, therefore, show a complete correspondence with the remain- 

 ing appendages, and, as was inferred from their nerve supply, 

 may perhaps be considered as prostomial and not metameric 

 appendages. 



1 More strictly metanauplius : the typical nauplius exhibits no segmenta- 

 tion of the trunk -region. 



