12 G-. O. Sars. 



and exhibit each at the base dorsally a small secondary 

 denticle not found in the previous stages. This denticle 

 represents the strong, spiniform inferior corner of the tail- 

 plates in the adult animal, whereas the terminal part of the 

 furcal processes becomes converted into the caudal claws. 



In this stage, moreover, the first trace of the compound 

 eye is seen above and somewhat in front of the ocellus. 

 It appears as 2 widely separated, and very small pigmentary 

 patches, each surrounded by a pellucid area, in which the 

 visual elements are in process of development. This organ 

 in the larva is accordingly double, whereas in the adult 

 animal the 2 halves are so completely fused together as to 

 constitute a simple mass. 



Finally, in this stage another organ has made its.first appear- 

 ance, viz., the heart, which is rather distinctly traceable in the 

 anterior part of the trunk dorsally (see fig. 1), exhibiting 

 already some irregular pulsations. 



In the structure of the Nauplian limbs, no perceptible 

 change has taken place, and the larva moves through the 

 water in the very same manner as in the previous stages. 



The length of the larva in this stage has increased to 

 0,52 mm. 



Sixth larval stage. 



(PL II, figs. 3. 4, 5). 



This stage is chiefly distinguished by the progressive 

 development of the carapace and of the branchial legs; 

 otherwise it nearly agrees with the preceding one, no exu- 

 viation having apparently taken place between the two. 



The carapace has now considerably increased in size, 

 and is rather broad and expanded, obtecting, like a mantle, 



