182 CRUSTACEA—-EUCARIDA—DECAPODA CHAP. 
tion to this rule is found in the Dromiacea, the most primitive 
of the Brachyura, to be soon considered, in which not only the 
third maxillipede, but also the first pair of pereiopods may be 
developed as biramous oars, a condition taking one back to the 
Fie. 124.—A, Zoaea, x 24, and B, Metazoaea, x 13, of Corystes cassivelaunus. Ab, 3rd 
abdominal segment ; An, Ist antenna; H, eye; G, gills; J, 1st maxillipede; 7.8, 
last thoracic appendage. (After Gurney.) 
Mysis stage of the Macrura. The Metazoaea (Fig. 124, B) has the 
rudiments of the thoracic limbs developed and crowded together 
at the back of the carapace; they are all laid down in their 
definitive forms, and the abdomen has the pleopods precociously 
developed. These Zoaeal stages are of course pelagic, but the Meta- 
zoaea next passes into the Megalopa stage (Fig. 125), in which 
the little crab forsakes its pelagic life and assumes the ground- 
habits of the adult; the Megalopa, which corresponds exactly to 
