In the crab, the young leaves the egg as a Zoæa (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 Zoæa, after it has acquired stalked eyes and a complete set of thoracic and abdominal members, and has passed into what is called the _Megalopa_ 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 Zoæa 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 _Penæus_, the rudiments of them only remain; in the lobster, they disappear altogether. Thus, in these animals, there is no difficulty in demonstrating that embryological uniformity of type of all the {284} limbs, complete evidence of which was not furnished by the development of the crayfish. In this crustacean, in fact, it would appear that the process of development has undergone its maximum of abbreviation. The embryo presents no distinct and independent Nauplius or Zoæa stages, and, as in the crab, there is no Schizopod or Mysis stage. The abdominal appendages are developed very early, and the new born young, which resembles the Megalopa stage of the crab, differs only in a few points from the adult animal. * * * * * Guided by comparative morphology, we are thus led to admit that the whole of the _Arthropoda_ are connected by closer or more remote degrees of affinity with the crayfish. If we were to study the perch and the pond-snail with similar care, we should be led to analogous conclusions. For the perch is related by similar gradations, in the first place, with other fishes; then more remotely, with frogs and newts, reptiles, birds, and mammals; or, in other words, with the whole of the great division of the _Vertebrata_. The pond-snail, by like reasoning upon analogous data, is connected with the _Mollusca_, in all their innumerable kinds of slugs, shellfish, squids, and cuttlefish. And, in each case, the study of development takes us back to an egg as the primary condition of the animal, and to the process of yelk division, the formation of a blastoderm, and the conversion of that blastoderm into a more or less modified {285} gastrula, as the early stages of development. The like is true of all the worms, sea-urchins, starfishes, jellyfishes, polypes, and sponges; and it is only in the minutest and simplest forms of animal life that the germ, or representative of the ovum becomes metamorphosed into the adult form without the preliminary process of division.