484 GEOLOGICAL SURVEY OF THE TERRITORIES. 
the width of the sac nearly amounts to the halfof its length, while they 
have an oval form in Art. milhausenii, the width of the bag nearly amount- 
ing to two-thirds of its length.' In ‘long continued domestication in salt 
water of gradually increased density I obtained, after several successive 
generations of Artemia salina, speciinens in which the gill-sacs and 
posterior branchial lobes had the same form and size as those of the 
specimens with the characters of A. milhauseniti, out of the Kujalniker 
Lake at 24° Beaumé, and in which still other characters appeared 
peculiar to them in free nature. 
It is important that in young individuals of A. salina in a certain age 
the gill-sacs and posterior branchial lobes have nearly the same size 
and form as in the mature individuals, with the characters of Artemia 
milhausenit, with the difference that in young individuals directly after 
quitting their larval state, and even, also, until they liberate themselves 
from the provisional parts of the second antenne, the largest of these 
appendages are not on the sixth pair of legs as in the mature forms, 
but on the fourth pair. Under the same circumstances under which in 
mature specimens of Artemia salina the gill-sacs on the fourth pair of 
legs amount in their length to the twenty-eighth and in their width the 
fifty-sixth part of the whole body-length, the gill-sacs in the young 
specimens (in the above-mentioned age) of the same pair of legs measure 
the seventeenth part of the body-length in length and the twenty- 
seventh part in their width; but at the time at which in mature speci- 
mens (at low temperature) of A. salina each gill-sac measured, on the 
sixth pair of legs, in its length the twenty-fourth, in its width the forty- 
eighth part of the body-length, in young specimens of the above-men- 
tioned age each gill-sac of the same pair of legs measured in its length 
the nineteenth and in its width the thirtieth part of the entire body- 
length. In young individuals of Artemia satina of this age the gill-saes 
of the eighth pair of legs corresponded, together with the posterior 
branchial lobes, in form and size with the same appendages of the same 
pair of legs in the mature individuals, which have the characters of A. 
milhausenit, inhabiting most saline water (about 24° Beaumé). In any 
case, on the whole, these appendages are, in the young specimens of 
A. salina of the stated age, considerably larger than in mature speci- 
mens of the same species, being, also, as it must be in the course of de- 
velopment, larger on the anterior pair of legs to the sixth than on the 
following pairs. In the young individuals of the age stated the gill-sacs 
measure on the third, fourth, and sixth pairs of legs in their mean length 
together the eighteenth and in their middle width the twenty-ninth part 
of the whole body-length, but in mature specimens of this species and 
under the same conditions the gill-saes of the third, fourth, and sixth 
pairs of legs measure in their middle length together only the twenty. 
eighth and in their middle width the fifty-sixth part.of the body-length- 
From the fact that the gill-sacs and posterior branchial lobes of the 
young individuals of Artemia salina of the stated age correspond in form 
and size with the same appendages in the mature individuals bearing 
the characters of A. milhausenit, we can infer that the latter is a gener- 
ation of A. salina retarded in its development in consequence of the ap- 
pearance of sexual maturity before the full development of the parts of 
the body. However, such an inference would be but partially true. 
The individuals with the characters of A. milhausenii not only exhibit 
retarded development under the influence of their surroundings, but 
they are also the result of the demand of the same element—the result 
‘Consult my report in Zeitschrift f. w. Zoologie, 1875, vol. xxv, Ist eae part, 
Tab. VI, figs. 7 and 8. 
