66 PRIMITIVE ANIMALS 



the thorax and the precocious development of the 

 abdomen and tail fan cannot possibly be primitive 

 features ; they are clearly specializations for the 

 pelagic free-swimming larval life, providing the larva 

 with suitable organs of locomotion while the develop- 

 ment of the middle region of the body may proceed. 

 It would be quite absurd to suppose that the ancestral 

 Malacostracan had a rudimentary thorax like a Zoaea, 

 because the Zoaea in this respect is clearly more 

 highly specialized and aberrant than any existing 

 adult Malacostracan. 



We have seen then that both the Nauplius and the 

 Zoaea do not represent adult ancestral forms, but 

 very ancient and ancestral larval forms. We cannot 

 however press the argument too far and say that no 

 larval form ever represents an adult ancestral form. 

 Every case must be judged on its own merits after 

 taking into account all the available evidence to be 

 derived from the comparative study of related forms. 

 Thus it seems indubitable that the so-called Mysis 

 larva (Fig. 14), a stage assumed by many higher 

 Malacostraca after the Zoaea, in which all the seg- 

 ments are fully formed and all the thoracic and 

 abdominal segments bear biramous appendages, does 

 very closely represent an adult ancestral form, the 

 actual existence of which is nearly realized by the 

 primitive Anaspides and Schizopod shrimps. Here 

 we are dealing with a late larval form, just before the 



