538 



ZOOLOGY 



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 (I-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 (#) 



Fig. 429. — Three stages in the development of Anus, 

 gland ; s. carapace ; 1 — U, cephalic appendagjs ; I- 

 (From Lang's Comparative Anatomy.) 



ft. frontal sensory organ ; L, digestive 

 -XII I, body -tegmenta and appendages. 



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 

 endopodite ; and a large outer branch, the exopoditc. 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 



