280 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



back in the course of its development to an ovum, and 

 that ovum gives rise to a blastoderm, from which the 

 parts of the embryo arise in a manner essentially similar 

 to that in which the young crayfish is developed. 



Moreover, in a large j^roportion of the Crustacea, the 

 embryo leaves the egg under the form of a small oval 

 body, termed a Nauplius (fig. 73, D), provided with 

 (usual!}') three pairs of appendages, which pla}' the part 

 of swimming limbs, and with a median eye. Changes of 

 form accompanied by sheddings of the cuticle take place, 

 in virtue of which the larva passes into a new stage, when 

 it is termed a Zocea (C). In this, the three pairs of loco- 

 motive appendages of the Nauplius are metamorphosed 

 into rudimentary antennules, antennae, and mandibles, 

 while two or more pairs of anterior thoracic appendages 

 provided with exopodites and hence appearing bifurcated, 

 subserve locomotion. The abdomen has grown out and 

 become a notable feature of the Zogea, but it has no 

 appendages. 



In some Podoplithalmia, as in Penceus (fig. 73), the 

 young leaves the egg as a Nauplius, and the Nauplius 

 becomes a Zosea. The hinder thoracic appendages, each 

 provided with an epipodite, appear ; the stalked eyes and 

 the abdominal members are developed, and the larva passes 

 into what is sometimes called the Mysis or ScJdzopod 

 stage. The adult state difi'ers from this chiefly in the 

 presence of branchiae and the rudimentary character of^ 

 the exopodites of the five posterior thoracic limbs. 



