484 GEOLOGICAL SUEVEY OF THE TEERITOEIES. 



the width of the sac nearly amounts to the half of its length, while they 

 have an oval form in Art milhcnisenii, 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 1 obtained, after several successive 

 generations of Artemia saUna, specimens 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. milhausenii, out of the Kujalniker 

 Lake at 24° Beaume, and in which still other characters appeared 

 X)eculiar to them in free nature. 



It is important that in young individuals of J., salinaina, 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 

 milhausenii, 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 antennse, 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 salina of this age the gill-sacs 

 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. 

 milhausenii, inhabiting most saline water (about 24° Beaume). 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 ou 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-sacs 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. 7nilhausenii, 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 interence would be but partially true. 

 The individuals with the characters of A. 7nilhausenii 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 



1 Consult my report in Zeitschrift f. w. Zoologie, 1875, voL xxv, 1st suppl. part, 



Tab. VI, figs. 7 and 8. 



