474 



DECAPODA. 



FIG. 218. MASTIGOPUS STAGE 

 OF SERGESTES. (From Claus.) 



Jiff"', maxilliped 3. 



helmet-shaped upper lip like 

 that of a typical Nauplius is 

 present, and the eyes are situa- 

 ted on very long stalks. 



In the true Zoasa stage there 

 appear on the five thoracic 



segments pouch-like birarnous rudiments of the limbs. The 

 tail becomes segmented ; but the segments, with the 

 exception of the sixth, remain without appendages. On 

 the sixth a very long bilobed pouch appears as the com- 

 mencement of the swimming feet of this segment. The 

 segments of the abdomen are armed with lateral spines. 



From the Zoasa stage the larva passes into the form 

 known as Acanthosoma, which represents the Mysis stage 

 of Penseus. The complex spikes on the dorsal shield of 

 the Zoaea stage are reduced to simple spines, but the 

 spines of the tail still retain their full size. In the appendages the chief 

 changes consist (i) in the reduction of the jointed outer ramus of the 

 second pair of antennas to a stump representing the scale, and the elon- 

 gation of the inner one to the flagellum ; (2) in the elongation of the five 

 ambulatory thoracic appendages into birarnous feet, like the maxillipeds, 

 and in the sprouting forth of rudimentary abdominal feet. 



