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. 73, 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 it is termed a _Zoæa_ (C). In this, the three pairs of locomotive appendages of the _Nauplius_ are metamorphosed into rudimentary antennules, antennæ, 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 Zoæa, but it has no appendages. In some _Podophthalmia_, as in _Penæus_ (fig, 73), the young leaves the egg as a Nauplius, and the Nauplius becomes a Zoæa. 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 branchiæ and the rudimentary character of the exopodites of the five posterior thoracic limbs. {281} [Illustration: FIG. 73. _Penæus semisulcatus._ A, adult (after de Haan. 1/2 nat. size); B, Zoæa, and C, less advanced Zoæa of a species of _Penæus_. D, Nauplius. (B, C, and D, after Fritz Müller.)] In the Opossum-shrimps (_Mysis_) the young does not leave the pouch of the mother until it is fully {282} developed; and, in this case, the _Nauplius_ state is passed through so rapidly and in so early and imperfect a condition of the embryo, that it would not be recognized except for the cuticle which is developed and is subsequently shed. [Illustration: FIG. 74. _Cancer pagurus._ A, newly hatched Zoæa; B, more advanced Zoæa; C, dorsal, and D, side view of Megalopa (after Spence Bate). (The figures A and B are more magnified than C and D.)] {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 Zoæa. In the lobsters, which have, throughout life, a large abdomen provided with swimmerets, the Zoæa, after going through a Mysis or Schizopod stage, passes into the adult form.