METAMORPHOSES OF THE CRUSTACEA. 283 



In the great majority of the Podophthalmia, the Nauplius 

 stage seems to he passed over without any such clear 

 evidence of its occurrence, and the young is set free as a 

 Zosea. In the lobsters, which have, throughout life, a 

 large abdomen provided with swimmerets, the Zoeea, 

 after going through a Mysis or Schizopod stage, passes 

 into the adult form. 



In the crab, the young leaves the egg as a Zoaaa 

 (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 Zoasa, 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 Zosea 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 Penceus, 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 



