THE BIOGENETIC LAW 



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becomes segmentecL 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 whicli 

 bear a pair of limbs apiece ; in the Branchiopods, on the other hand, tlie 

 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 



Fig. io8. Nauplius larva of one of the lower Crustaceans. After Fritz 

 Miiller. Ait, the frontal eye ; I, first pair of limbs, corresiDonding to the future 

 antennae ; II and. Ill, two biramose swimming appendages. 



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 antennae in the 

 Copepods and the jaws of the Branchiopods, Cirrhipedes, &c. 



If then we have before us in the ' germinal history ' ( ontogeny ) 

 a fairly precise repetition of the ' racial history ' (phylogeny), we may 

 deduce from this that the primitive forms of the Crustacean race 

 were animals which consisted of few segments, and that from these, 

 in the course of the earth's history, the very diverse modern groups 

 of Crustaceans have arisen, by the addition of new segments, and the 

 adaptation of the limbs upon them, which were at first biramose 



II. M 



