^' fllUHTACK A— ONTOGENY 383 



A. The laxval history of the Crustacea— The Development of Apus 

 (Order Phyllopoda, Family Branohiopoda). 



1st Larval Stage, NaupUus.—Owt of the egg is hatched an oval larva narrowing 

 like a pear posteriorly, with a median frontal eye and three pairs of limhs, the most 

 anterior of which is rod-shaped, while the two posterior are biramose. Its form and 

 its setfe are illustrated in Fig. 257, A. 



On the dorsal side of the body the dorsal shield has begun to form. The anus 

 lies in an indentation of the posterior margin of the body. This first stage is called 



Fig. 257.— Larvae of Apus (after Claus). A, Naupllus, just hatched, mtli the rudiments of the 6 

 anterior trunk segments I-IV. U, 2d larval stage, with the rudiments of the anterior maxillffi and 

 the first 7 trunk segments. C, 4th larval stage, i. Liver ; s, shell ; fs, frontal sensoiy organ. 



the Ncniplius larva : it is met with in essentially the same form as the first product of 

 the egg in many Crustaceans ; we shall therefore enumerate its general characteristics. 

 Body unsegmented with median frontal eye, with dorsal shield and frontal 

 sensory organs (filaments, etc.) with 3 pairs of limbs, the first of which is simply 

 rod-shaped, i.e. consists of a single row of joints, while the 2 posterior are 

 biramose, i.e. consist of a protopodite, endopodite, and exopodite (shaft, inner 

 and outer branch). In all Crustaceans the first pair of limbs of the Nauplius 

 become the anterior antennas, the second the posterior antennse, the third the 

 mandibles of the adult animal. 



The Nauplius of Apus (Fig. 257, A) is distinguished from the typical Nauplius 



