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, 
XIZ) 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, X777). 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 maxillw. 
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 (Wig. 
11) present a little lobe on the inner side (7%), which is the beginning 
of the formation of the inner branch. 
The telson (Fig. 13, ¢) is at this stage narrower behind than in front. 
Its hind margin is armed with cight sete. 
Length, 5.5 mm. 
Sixth Larval Stage (Pl. YII. Figs. 14-16; Pl. 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 fect. 
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 antenne (Pl. III. Fig. 14, v 2) 
is clearly divided into six or seven segments. The chele (Pl. IIL. 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 (Pl. IIT. Fig. 18, t) is cut into a shallow notch, and bears 
three pairs of long sete. In the interval between the central pair are 
two much smaller sete. 
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 go little from the one just noticed (no new 
structures appearing), that I hate included all three under the name of 
sixth larval stage. 
