294 



THE CRUSTACEA 



have already begun to outstrip these in order of development and 

 are larger and bilobed. In a later Zoea-stage (Fig. 171, C) the 

 sixth pair form with the furcate telson a well-marked tail-fan, but 

 the first five pairs of abdominal limbs are stated to be temporarily 

 suppressed, to reappear again at a later stage ; a retrograde change 

 is also observed in the peduncle of the antennule, which in the 

 later Protozoea was divided into five segments but now becomes 



Fio. 171. 



Later stages of Penaeus. A, older Protozoea-stage. B, under-surface of thorax and 

 abdomen of somewhat later stage with rudiments of limbs. C, Zoea-stage. D, Schizopod-stage. 

 1, antennule ; 2, antenna; 3, mandible ; 4, maxillula; 5, maxilla ; I-VII I, thoracic appendages ; 

 (IV-VIII), the posterior thoracic somites; fij-ctg, pleopods ; <?, uropods ; nb, abdomen; ei., 

 endopodite ; ex, exopodite ; fr, frontal sense-organ ; /,, hepatic caeca ; t, telson. (After 

 Claus, from Korschelt and Heider's Embryology.) 



once more unsegmented. The five posterior pairs of thoracic 

 limbs (legs), which in this stage are bilobed rudiments, develop in 

 the succeeding Sckizopod-st&gQ (Fig. 171, D) (usually called the 

 Jfysis-stage) into biramous natatory limbs and take up the function 

 of locomotion hitherto fulfilled chiefly by the antennae. The 

 abdomen has now increased greatly in size as compared with the 

 cephalothorax, and the first five pairs of abdominal appendages 



