[i)] THE FIKST FOOD OF 'i'lIE COMMON WrilTFFlSH. 779 



exteiide«l from the tips oi" the pectorals on the belly to ;i point oi)positc 

 the middle of the same fius on the back. In most the egg-sac did not 

 protrude externally, being reduced in som6 to a droplet of oil, but re- 

 maining in a few of a size at least as great as that of the head. The 

 alimentary caual was, of course, a simple straight tube, without any dis- 

 tinction of stomach and intestine. 



The sufferings of these fry in transit had doubtless weakened the 

 vitality of the survivors, and although every care was taken to keei> the 

 water of the tank fresh and pure, about one-third of those remaining 

 died during the progress of the experiment. The aquarium in which 

 they were confined was built of glass, and had a capacity of about one 

 hundred cubic feet. The temperature, tried repeatedly, stood at 42<^ F. 

 A steady current of the water of the lake was maintained through 

 this tank, entering through a rose, from which it fell in a spray, thus 

 insuring perfect aeration. 



By far the greater part of the organic contents of the water of 

 the lake, as shown by the product of the towing-net, consisted of 

 diatoms i>i immense variety, which formed always a greenish mu- 

 cilaginous coating upon the inner surface of the muslin net. In this 

 were entangled a variety of rotifers, occasional filamentous Algai, and 

 many Entomostraca, the latter belonging chiefly to the genera Cyclops, 

 Diaptomus, and Limnocalanus among the Copepoda, and to Daphnia 

 among the Cladocera. 



As the Entomostraca proved to be far the most important elements 

 of this food supply, the particulars respecting them may be properly 

 more fully given. The smallest of all was a Cyclops, then new, but 

 since described by me under the name of Cyclops thomasi* This little 

 Entomostracan is only .04 inch long, by .011 wide. The next in size, 

 and by far the most abundant member of this group, was a Diaptomus, 

 likewise new, described in the paper just cited, under the name of 

 Diaptomus sicilis. This appears in two forms, one evidently young in 

 the stage just preceding the adult. Full-grown individuals were .065 

 inch long by one-fourth that depth. The Limnocalanus was a much 

 larger form, evidently preying, to a considerable extent, upon the two 

 just mentioned. All the Cladocera noticed were Daphnia hyalina, an 

 elegant and extremely transparent species, occurring likewise in the 

 lakes of Europe. A single insect larval form (Chironomus) should 

 likewise be mentioned in this connection, since it had about the same 

 size and consistence of the Entomostraca, and was consequentlj' equally 

 available for food. 



The specimens of each of the above species from a certain quantity 

 of these collections were counted, in order to give a definite idea of 

 their relative abundance in the lake. The Diaptomus nuudiered 225, 

 the Cyclops 75, Limnocalanus 7, Daphnia 3, and Chironomus larvj© 1. 



*0a some Entomostraca of Lake Michigan and adjacent waters. American Natu- 

 ralist, Vol. XVI., No. VIII, Angnst, lP8t>. pp. filO and M9. 



