214 



INVERTEBKATA 



CHAP. 



legs, have enormously long endopodites and short outer forks 

 (exopodites). 



Here we see, as has been repeatedly emphasized in this book, that 

 Nature treats as a single organ an apparatus consisting of several 

 pairs of metamerically arranged organs, which co-operate in the 

 performance of a single function ; and when the whole organ is 

 diminished in size as when it is reproduced in a larva the number 

 of component similar organs is reduced also. 



This modified Mysis is known as a Phyllosoma or glass-crab. As 



FIG. 161. "Phyllosoma" larva of Palinurus mil (jar is, ventral view. 

 (After Cunningham.) 



(ili, abdomen ; hep, liver saccules seen through ex, exopoclite of thoracic appendages. 



it grows and moults the first maxillipede grows larger, and the last 

 two pairs of thoracic legs also make their appearance ; so also do the 

 appendages of the abdomen, and thus the adult condition is approached. 

 The explanation of the singular appearance of this larva is, that in 

 the case of the Loricata the Mysis has ceased to be an actively-swim- 

 ming organism and has become a surface-drifter ; the long legs being 

 widely spread out and acting as supports. 



Another series of modifications of the Mysis stage have been 

 described by Sars (1891) in the case of the Craugonidae amongst 

 Caridea. The Zoaea of these forms it will be remembered has, 

 when hatched, three pairs of maxillipedes developed as forked swim- 



