TYPICAL FORM OF THE CRUSTACEAN LIMBS. 



193 



in the most varied groups of Crustacea. This general occurrence of the Nauplius 

 in the ontogeny of all Crustacea shows that the hypothetical common ancestor 

 had already passed through the stage at which the Nauplius was acquired, 

 and that therefore the modification in the ontogeny of the Crustacea just 

 described took place in very early times. This appears the less surprising 

 when we realise how greatly the ontogeny of a form is influenced by variation 

 affecting the adult. Only in so far as the wide occurrence of the Nauplius 

 larva justifies us in forming conclusions as to the ontogeny of the racial form 

 of the Crustacea, can a certain phylogenetic significance be ascribed to it 

 (HATSCHEK, Lehrbuch der Zool., p. 25). 



The further development of the Nauplius stage is accomplished in 

 the Entomostraca, e.g., in the Phyllopoda and also in some Copepoda, 

 by a series of very gradual changes of shape through many moults, 

 the adult condition being reached by a continuous increase in the 

 number of body-segments and of 

 pairs of limbs, the growth of the 

 dorsal shell-fold, the appearance 

 of the lateral eye, and other 

 transformations occurring one 

 -after the other. While, in these 

 lowest forms, the course of 

 metamorphosis is comparatively 

 simple, it becomes more compli- 

 cated among the Malacostraca, 

 where independent larval forms, 

 notably the Zoaea, are inter- 

 calated. These do not belong to 

 the direct series of transforma- 

 tions between the Nauplius and 

 the adult form, but are dis- 

 tinguished by secondarily acquired peculiarities, resulting in a great 

 increase in number of larval forms. (For the metamorphoses of 

 the Malacostraca, see p. 246.) 



FIG. 90. A, thoracic limb of a Copepod 

 (after GLAUS). B, abdominal limb of Gam- 

 marus locusta (after BOAS). 1, first, 2, second 

 joint of the protopodite ; en, endopodite : 

 ex, exopodite. 



2. Typical Form of the Crustacean Limbs. 



In the two posterior pairs of Nauplius appendages, we have a 

 very primitive form of Crustacean limb. With the exception of 

 the first antenna (antennule) which shows an altogether hetero- 

 morphous structure, all the Crustacean limbs can be traced back to 

 -a fundamental type corresponding to the biramose second and third 

 pairs of Nauplius limbs. We can always distinguish a proximal 

 section as the stem of the limb or protopodite (HUXLEY), which 



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