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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVIL 



egg", and comes forth a miniature adult, while the anieriea/ni!^ larva 

 spends onh' sixteen or seventeen daj^s in the egg and comes forth in a 

 typical Cyclops stage to undergo many subsequent transformations 

 before reaching the adult form. 



The temperature of the aquarium was about 72° F., or a few 

 degrees warmer than our fresh-water ponds during the summer 

 season. The eggs which were laid June 8 began to hatch the 25th, a 

 period of onl}^ seventeen days, about half the length of time required 

 by the eggs of ^1. foUacenx and catostomi and only one-fifth of that 

 required by -i. stizostethii and mtgaJojM. 



The newly hatched larvae (tig. 16) are not as lively as those of 



inegalopti^ which woidd be ex- 

 pected from the fact that they 

 are not proxided with as good 

 organs of locomotion, but they 

 are also more sluggish than the 

 vatoxtomi larva% which have ex- 

 actly the same structure. Fur- 

 thermore, when first hatched 

 they stick to the bottom of the 

 dish and do not swim up and 

 toward the light until the third 

 or fourth day. And when they 

 do get up to the surface they 

 prove to be negatively helio- 

 tropic, in sharp contrast with all 

 the other Argulus larvje thus far 

 studied, which are positively 

 heliotropic to a very marked 

 degree. 



Owing both to the position and 

 incomplete development of the 

 temporary locomotor organs, the 

 motion of these nauplii is jerky 

 and cyclops-like. They are not 

 as transparent as the other larva^ and do not furnish as favorable 

 objects for study, owing to the presence of a large area of rust-colored 

 pigment over the stomach and anterior intestine which hides much of 

 the internal anatomy. 



In general shape they are short and broad, the carapace being well 

 rounded anteriorly, with a broad and shallow posterior sinus, while the 

 free thorax and abdomen form a broad triangle, relatively shorter 

 than that oifoliaceus and catostomi . Around the anterior margin of 

 the carapace can be seen large tactile hairs similar to those found 

 upon other species. But they are much fewer in number and more 

 scattered than in megaJojJs, and the intervening cilia are very minute 



17. — Free thorax axd abdomen of argulx's 

 americanus larva. 



