METAMORPHOSIS OF CORYSTES OASSIVELAUNUS. 469 



Tliii'd Stage. — Measurements (average and range of 

 variation in eleven specimeus) : 



1. Length of rostrum 



2. „ dorsal spine 



3. Tip to tip of spines 



4. Length of body . 



5. ,, thorax 



6. Ratio of 4 to 3, 1 : 1-53. 



7. Length of third pleo- 



pod . . . . -25 „ ... -2 „ -35 „ 



8. Length of fourth abdo- 



minal segment . . '41 „ ... 'o „ '45 „ 



At this stage the reduction in length of the dorsal and 

 rostral spines as compared with the total length of the body- 

 has become very noticeable^ and the increased completeness 

 of the development of the limbs shows an evident approach 

 towards the Megalopa. 



The differences between the zogea at this period and that 

 of Stage 2 consists more in the increased development of 

 parts already formed than in the acquisition of new ones. 

 The internal branch of the first antenna is developing^ and 

 the base of the antenna shows a certain degree of swelling in 

 preparation for the formation of the auditory organ. In the 

 second antenna the increase in length of the flagellum is very 

 striking, and it is now about twice the length of the exo- 

 podite, showing traces, beneath the cuticle, of segmentation. 

 The mandibular palp is present as a small two-jointed process. 



In the first maxilla no change has taken place, but in the 

 second maxilla (fig. 11) the scaphognathite has not only 

 increased in size, but is provided with a very greatly in- 

 creased number of setie, which fringe its edge, and are of 

 more or less uniform size. Those of the posterior border are 

 not lono'er or stouter than the rest. These chani>-es are 

 probably associated with the further development of the 



