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 proportion of the Crustacea, the 
embryo leaves the egg under the form of a small oval 
body, termed a Nauplius (fig. 78, D), provided with 
(usually) three pairs of appendages, which play 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 
itis termed a Zowa (C). In this, the three pairs of loco- 
motive appendages of the Nauplius are metamorphosed 
into rudimentary antennules, antenne, 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 Zowxa, but it has no 
appendages. 
In some Podophthalmia, as in Peneus (fig. 78), the 
young leaves the egg as a Nauplius, and the Nauplius 
becomesa Zowa. 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 Schizopod 
stage. The adult state differs from this chiefly in the 
presence of branchie and the rudimentary character of 
the exopodites of the five posterior thoracic limbs. 
