538 



ZOOLOGY 



SECT. 



The embryo is hatched in the form shown in Fig. 429, A. The 

 body is oval, and is divisible into three regions a large anterior or 

 head-region ; an intermediate trunk-region, the hinder part of which 

 already shows signs of segmentation (/- V) and a posterior bilobed 

 anal region. The head-region bears a single median eye, and a 

 pair of small unjointed appendages (7), each with two large setae 

 at its extremity : these become the antennules of the adult. The 

 trunk region bears two pairs of appendages, the first of which (2) 



FIG. 429. Three stages in the development of Apus. /s. frontal sensory organ ; L, digestive 

 gland; s. carapace; 1 !,, cephalic appendages; I XIII, body -segments and appendages. 

 (From Lang's Comparative Anatomy.) 



is very large and fringed with setae, but is chiefly remark- 

 able for being biramous or two-branched being formed of a 

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

 cndopodite ; and a large outer branch, the cxopodite. 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 



