64 PRIMITIVE ANIMALS 



Entomostracan Nauplius. This is a striking example 

 of Von Baer's law and it tells us that the common 

 ancestor of the Entomostraca and Malacostraca 

 probably had a Nauplius larva. But it is not in the 

 least justifiable to deduce, as some have attempted, 

 that the Nauplius larva represents the adult ancestral 

 form from which the Crustacea as a whole have 

 sprung. The possession of only three segments and 

 of the simple unfaceted eye, which are the really 

 characteristic features of the Nauplius larva, do not 

 carry us a degree nearer the common Arthropodan 

 type from which the Crustacea, together with the 

 Arachnids, Insects and Myriapods, must originally 

 have diverged. Everything shows that such an 

 ancestral type must have possessed numerous, prob- 

 ably similar, segments, and at least the beginnings of 

 the compound faceted eyes, which occur under some- 

 what different forms in all the great Arthropodan 

 classes. 



There is another instructive Crustacean larva, the 

 Zoaea (Fig. 13), characteristic of the Malacostraca, 

 which, while admirably illustrating the truth of Von 

 Baer's generalization, cannot be accepted as repre- 

 senting the type of the adult Malacostracan ancestor. 

 The Zoaea larva is characterized by the possession of 

 two pairs of antennae, mandibles, two pairs of maxillae 

 and two biramous maxillipedes. The hinder part of 

 the thorax is in a suppressed or retarded condition of 



