Given a body divided into somites, each with a pair of appendages; and given the power to modify those somites and their appendages in strict accordance with the principles by which the common plan of the Podophthalmia is modified in the actually existing members of that order; and the whole of the Arthropoda, which probably make up two-thirds of the animal world, might readily be educed from one primitive form.
And this conclusion is not merely speculative. As a matter of observation, though the Arthropoda are not all evolved from one primitive form, in one sense of the words, yet they are in another. For each can be traced {280} 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. [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}
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.
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.)
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