l82 



CRUSTACEA EUCARIDA DECAPODA 



tioii to this rule is found iu 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 



Fig. 124. — A, Zoaea, x 24, and B, Metazoaea, x \Z, oi Corystes cassivdminus. Ah,ZvCi 

 abdominal .segment ; An, 1st antenna ; E, eye ; G, gills ; M, 1st maxillipede ; T.8, 

 last thoracic appendage. (After Gurney.) 



Mysis stage of the Macrura. The Metazoaea (Fig. 1 24, 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- 

 liabits of the adult ; the Megalopa, which corresponds exactly to 



