MUSEUM OF COMPARATIVE ZOOLOGY. 317 



(Fig. 4, IX). The chela of the following pair of appendages is not so 

 much developed as yet. The penultimate pair of thoracic legs (Fig. 4, 

 XII) are still rudimentary, unsegmented, double sacs. 



The last pair of thoracic legs, however, are fully developed as long, 

 simple, 5- or 6-jointed appendages (Fig. 4, XIII). This pair alone of 

 all the thoracic appendages are destitute of external branches from 

 their very earliest appearance. They are generally borne curved for- 

 ward under the sternum, in which position the distal segment (which 

 bears a long, coarse terminal bristle) reaches forward to the maxillae. 

 The inner branches of the double thoracic legs are in the same way 

 tucked under the breast when the animal swims. 



The appendages of the first, second, third, fourth, and fifth abdominal 

 somites have gained in size. The second, third, and fourth pairs (Fig. 

 11) present a little lobe on the inner side (ri), which is the beginning 

 of the formation of the inner branch. 



The telson (Fig. 13, t) is at this stage narrower behind than in front. 

 Its hind margin is armed with eight setae. 



Length, 5.5 mm. 



Sixth Larval Stage (PI. III. Figs. 14 - 1G ; PI. IV. Figs. 1-8). — The 

 larva issues from the next moult provided for the first time with all the 

 swimming-legs. The sacs which represented the penultimate pair pre- 

 vious to the moult have set free a pair of two-branched limbs similar to 

 those in front, but with quite small external branches. This is the 

 nearest approach to a true Mysis-stage which appears in the develop- 

 ment. It differs from the Schizopod in the lack of the outer branch of 

 the last thoracic feet. 



Besides the unfolding of the penultimate pair of thoracic appendages, 

 there is but little change from the phase immediately before the last 

 moult. The inner branch of the second antennas (PI. III. Fig. 14, r i) 

 is clearly divided into six or seven segments. The chela? (PI. III. Figs. 

 15, 16) are more perfectly formed. The two lobes of the abdominal 

 appendages (PI. III. Fig. 17) have become enlarged and marked off by 

 constriction from the basal portion of the limb. The posterior border 

 of the telson (PI. III. Fig. 18, t) is cut into a shallow notch, and bears 

 three pairs of long seta?. In the interval between the central pair are 

 two much smaller seta?. 



Length from tip of the rostrum to end of the telson, 6 mm. 



I succeeded in rearing this stage through two successive moults. 

 The resulting' forms differed so little from the one just noticed (no new 

 structures appearing), that I have included all three under the name of 

 sixth larval stage.. 



