For the establishment of these views Fritz Müller relied chiefly upon larvæ, and in particular upon those of Crustaceans, and the facts, which were in part new and in part interpreted in a new manner, were so striking that it was impossible to deny their importance. In particular, he drew attention to the fact that in several of the lower orders of Crustaceans the most diverse species have a similar form when they leave the egg, all of them being small, unsegmented larvæ, with a frontal eye and a helmet-like upper lip, and with three pairs of appendages, the two posterior pairs being two-branched swimming-legs beset with bristles. In the size and form of the body, and especially of the chitinous carapace, these larvæ differ in the various systematic groups; thus, for instance, the larvæ of the Copepods are simply oval, while those of the Cirrhipedes are produced anteriorly into two horn-like processes, and so on, but in essentials they are all alike, and for a long time these larval forms had been distinguished by the special name of 'Nauplius' (Fig. 109). The development of the perfect animal begins with the longitudinal growth of the Nauplius; the posterior end lengthens and becomes segmented, between the anterior portion and the tail more segments are interpolated, and on these new pairs of limbs may grow. The number of these segments and limbs varies according to the group to which the animal belongs. Thus the body of the perfect animal in the little Cyprids always consists of eight segments, seven of which bear a pair of limbs apiece; in the Branchiopods, on the other hand, the number of segments varies from twenty to sixty, with ten to over forty pairs of legs; in the Daphnids or water-fleas there are about ten segments, with seven to ten pairs of limbs, and in the Copepods about seventeen segments with eleven pairs of limbs. The difference between the orders depends not only upon the differences in the number of segments and limbs, but quite as much upon the form and development of the segments, and above all of the limbs, and in this connexion it is worthy of note that the additional limbs which grow out usually appear at first as biramose swimming-legs, and are subsequently modified in form. Thus the pairs of jaws, three in number, which appear in the Copepods are developed from such swimming-legs, and so also is the second pair of antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes, &c. [Illustration: FIG. 108. Nauplius larva of one of the lower Crustaceans. After Fritz Müller. _Au_, the frontal eye; _I_, first pair of limbs, corresponding to the future antennæ; _II_ and _III_, two biramose swimming appendages.]