496 



ZOOLOGY 



SECT. 



Development. The eggs are centrolecithal, i.e., have an 

 accumulation of yolk in the centre surrounded by a superficial 

 layer of protoplasm. The process of segmentation and the forma- 

 tion of the germ-layers, has not been observed. 



The embryo is hatched in the form shown in Fig. 395, 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 



Fio. 395. Three stages in the development of Apus. ./>. frontal sensory organ ; L, digestive 

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

 (From Lang's Comparative Anatomy.) 



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

 f iKt I region. The head-region bears a single median eye, and a 

 pair of small unjointed appendages (1), each with two large setae 

 at its extremity, which become the antennules of the adult. The 

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

 is very large and fringed with setaa, 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 exopodite. This second 



