METAMORPHOSES OF THE CRUSTACEA. 283 
In the great majority of the Podophthalmia, the Nauplius 
stage seems to be passed over without any such clear 
evidence of its occurrence, and the young is set free as a 
Zora. In the lobsters, which have, throughout life, a 
large abdomen provided with swimmerets, the Zoea, 
after going through a Mysis or Schizopod stage, passes 
into the adult form. 
In the crab, the young leaves the egg as a Zoma 
(fig. 74, A and B). But this is not followed by a 
Schizopod stage, inasmuch as the five hinder pair of 
thoracic limbs are apparently, from the first, devoid of 
exopodites. But the Zowa, after it has acquired stalked 
eyes and a complete set of thoracic and abdominal 
members, and has passed into what is called the Mega- 
lopa stage (fig. 74, C and D), suffers a more complete 
metamorphosis. The carapace widens, the fore part of 
the head is modified so as to bring about the formation 
of the characteristic metope: and the abdomen, losing 
more or fewer of its posterior appendages, takes up its 
final position under the thorax. 
In the Zowa state, those thoracic limbs which give rise 
to the maxillipedes are provided with well-developed 
exopodites, and in the free Mysis state all these limbs 
have exopodites. In the Opossum-shrimps these persist 
throughout life; in Peneus, the rudiments of them only 
remain; in the lobster, they disappear altogether. 
Thus, in these animals, there is no difficulty in demon- 
strating that embryological uniformity of type of all the 
