XI PHYLUM ARTHROPODA 539 



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 nauplins} 



The nauplius swims freely, chiefly by vigorous strokes of 

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

 moults or ccdyses, the cuticle being cast off and the animal 

 emerging in the form shown in t'ig. 429, B. The trunk-region has 

 elongated, new segments having been added, as in Cha3topods, 

 between those previously present and the anal region. The 

 antennules have become shifted backwards, and rudiments of a 

 fourth pair of appendages, the first maxillse (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. 429, 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 seg- 

 ments are added with their appendages, the antennules and 

 antennae degenerate — the latter sometimes disappearing alto- 

 gether, — 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 sujiports 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. 



h. The Fresh-water Cra3'fish {AsUicus fluviatilis). 



Astricus JluvicUilis is common in streams and rivers in England 

 and the continent of Europe ; allied species occur in Asia and 

 North America; and fresh-water Crayfishes belonging to other 



1 More strictly metanaupliut : the typical nauplius exhibits no segmentation 

 of the trunk region. 



