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 setae 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 th.e 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 

 chehe 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. 



Length of the fifth larva 



Distance from tip of extended chelipeds to end of telson 



Length of flagellum of second antenna 



Length of carapace in third stage 



Length of carapace in fourth stage 



Length of carapace in fifth stage 



Greatest width of carapace, fifth stage 



Length of large chelae, fourth stage 



Length of large chelae, fifth stage 



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 chromatophores. 



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 



