CRUSTACEA. 



475 



The most obvious external indica- 

 tions of the passage from the Acantho- 

 soma to the Mastigopus stage (fig. 218) 

 are to be found in the elongation of the 

 abdomen, the reduction and flattening 

 of the cephalo-thoracic shield, and the 

 nearly complete obliteration of all the 

 spines but the anterior. The* eyes on 

 their elongated stalks are still very 

 characteristic, and the elongation of 

 the flagellum of the second pair of 

 antennae is very striking. 



The maxillae and maxillipeds un- 

 dergo considerable metamorphosis, the 

 abdominal feet attain their adult form, 

 and the three anterior thoracic ambu- 

 latory legs lose their outer rami. The 

 most remarkable change of all concerns 

 the two last pairs of thoracic appen- 

 dages, which, instead of being meta- 

 morphosed like the preceding ones, are 

 completely or nearly completely thrown 

 off in the moult which inaugurates the 

 Mastigopus stage, and are subsequently 

 redeveloped. With the reappearance 

 of these appendages, and the changes 

 in the other appendages already indi- 

 cated, the adult form is practically 

 attained. 



FIG. 219. LARVA OF HIPPOLYTE 

 IN ZO^EA STAGE. (From Claus.) 



MX', and MX", maxilla i and i ; 

 Mf. Mf. Mf". maxillipeds. 



FIG. 220. OLDER LARVA OK HIPPOLYTE AFTER THE THORACIC APPENDAGES HAVE 

 BECOME FORMED. (From Claus.) 



