VIII 



ABTHKOPODA 



211 



polychaete worm into the shrimp does the Zoaea represent ? 

 Obviously one in which (1) the first antenna had become purely 

 sensory, (2) the second antenna had moved in front of the mouth 

 and had lost its masticatory function, (3) the mandible had become 

 purely masticatory, (4) the next two pairs of appendages (maxillae) 

 had been modified into jaws, (5) the main swimming function had 

 been thrown on the first two or three appendages of the thorax, 

 and, (6) compound eyes and a carapace had been developed. 



If we read over this list we might conclude that the Phyllopod 

 genus Apus, if it had possessed better developed antennae, would have 

 given us a good idea of what the ancestor represented by the Zoaea 

 looked like. The first two pairs of thoracic appendages of Apus are 

 developed into long antenna-like organs. All the others, of which 



d.lsp 



rnxi 



Fi. 158. Zoaea larva of Porcellane longicornia, after the first moult. (After Sars. ) 



d.l.sp, dorso-lateral spines of the carapace ; r, enormously elongated rostral spiue ; 

 th, rudiments of hinder thoracic appendages. 



there is a great number, are thin and parapodia-like. These, which 

 must have existed in the ancestor, are not represented in the Zoaea, 

 owing doubtless to their physiological unimportance and the diminu- 

 tion in size of the larva. We cannot, however, well imagine that the 

 abdomen in the ancestor was devoid of appendages, although it is so 

 in most Zoaeae. In the Zoaeae of the Penaeidea, indeed, the append- 

 ages of the last segment are developed, and there are vestiges of 

 appendages on the other segments of the abdomen. 



The abdomen, as a region with peculiar appendages, is character- 

 istic of the Malacostraca, and the physiological necessity which led 

 to its evolution can be inferred by watching the way in which it is 

 used by Zoaeae. Many of these swim on their backs, using the long 

 spines which project from the carapace as a keel. The maxillipedes 

 are used as oars, and the abdomen functions as a rudder. 



We can now form to ourselves a picture of the course which 

 evolution followed in transforming the ancestor represented by a 



