i 
r 
176 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
completely covered with foreign matter that they could hardly swim, some lying upon 
their backs on the bottom of the aquarium. Colonies of zoothamnium-like protozoa 
were clustered over all their appendages. The set;e were loaded with sediment filled 
with bacteria, diatoms, and infusoria. This illustrates the fate which awaits the larvae 
of all Crustacea, when crowded in small aquaria. 
The fourth larval stage lasted (in the average of nine individuals, which were 
raised from the egg) 13 days and varied from 10 to 19 days. 
THE FIFTH STAGE. 
In fifteen individuals known to have molted five times, the average length was 
14.2 mm., and the extremes 13.4 and 15 mm. 
There are no external marks by which the fifth stage can be distinguished from 
the fourth or even from the sixth stage with any degree of precision, at least by the 
unaided eye (fig. 31, plate 18). Neither the size nor color changes can be invariably 
relied upon. Microscopical examination, however, shows that the rudiments of the 
swimming exopodites, which could be readily detected in the fourth larva, have now 
become still more reduced, while in the sixth stage they have completely disappeared. 
The following notes illustrate the changes of color which are observed in larvae 
passing from the fourth to the fifth and sixth stages. On July 7 a fourth larva 
(No. 35, table 34) showed the typical colors, reddish-brown and various tints of green. 
When observed eight days later the color was dark maroon. The fifth molt occurred 
about July 17 (length, 14.8 mm.). The color was then greenish-brown; the large 
chelae reddish-brown, tipped with cream color, most marked upon the propodi. As 
in some fourth larvae, there is a terminal light spot on the exopodite of the uropod. 
Faint light spots are also seen on the sides of the abdominal segments. There are, 
moreover, two very prominent, white, discoidal areas on the carapace corresponding to 
the insertions of muscles, as already pointed out. 
The following measurements of this larva will give a clearer idea of the length 
of some of the parts and of their increase after the molt: 
Measurements of larva, third to fifth stages. 
Millime- 
ters. 
14. 8 
Distance from tip of extended chelipeds to end of telson 
19.5 
11 
5.4 
6.3 
7 
3. 1 
4 
5 
Another larva raised from the third stage (No. 12, table 34) is olive-green, with 
the characteristic white marks very faint on the carapace. The large chelae are 
yellowish-green, due to the presence of blue and yellow cliromatophores. 
In the case of a fifth larva reared in an aquarium the colors resemble those of 
the sixth stage, represented in plate 25. The white spots on the carapace have the 
disposition shown in fig. 37, plate 24. The third pair of maxillipeds are tipped with 
white. 
No noticeable differentiation can be detected in the large claws unless occasioned 
by loss and subsequent growth or by injury to one of the members. It often happens 
