THE DECAPODA 



295 



begin to reappear. The various appendages now begin to assume 

 the form which they have in the adult. The antennules have a 

 three-segmented peduncle, with two flagella as yet unsegmented. 

 The endopodite and exopodite of the antenna become respectively 

 flagellum and scale. The palp of the mandible begins to redevelop. 

 In a later stage, which may be called post-larval, the exopodites of 

 the thoracic limbs become reduced and the abdominal appendages, 

 now well developed, take on the function of swimming-organs. 



While it is tolerably certain that the general course of develop- 

 ment in the Penaeidae is as described above, it is to be observed 

 that as yet the complete series of larval forms has not been traced 

 out in the case of any one species, and it is just possible that some 

 of the changes stated to occur, e.g. the alleged temporary disappear- 



A n 



Fio. 172. 



Metanauplius-stages of Leucifer. A, just hatched ; B, later stage, a', antennule ; a", 

 antenna ; d, shell-fold ; ml, mandible ; mf, first maxilliped ; iraf, maxilltria ; mx", maxilla ; 

 o7, labrum ; p, paragnatha. (After Brooks, from Korschelt and Heider's Embryology.) 



ance of the first five abdominal appendages in the later Zoea-stage, 

 may be due to confusing together in one series the larvae of 

 different species. In the closely related family of the Sergestidae, 

 however, Brooks has been able to trace out in considerable detail 

 the life-history of a single species, Leucifer typus. In this case the 

 animal leaves the egg as a metanauplius (Fig. 172, A) with four 

 pairs of limb -buds already visible behind the three pairs of 

 nauplius-limbs. This is followed by a later metanauplius (Fig. 172, 

 B) in which the shell-fold and the masticatory process of the 

 mandible appear. The Protozoea (Fig. 173, A), with seven pairs 

 of functional limbs, differs from that of Penaeus chiefly in the 

 different shape of the carapace, which has already the beginning 

 of a rostrum, and in having only four of the six posterior thoracic 



