252 



THE RATE OF GROWTH 



[CH. 



centration of the brine (partly during the hfe-time of the individual, 

 but more markedly during the short season which suffices for the 

 development of three or four, or perhaps more, successive genera- 

 tions), it is found that the tail comes to bear fewer and fewer 

 bristles, and the tail-fins themselves tend at last to disappear: 

 these changes corresponding to what have been described as the 

 specific characters of A. milhausenii, and of a still more extreme 

 form, A. koppeniana] while on the other hand, progressive dilution 

 of the water tends ' to precisely opposite conditions, resulting in 

 forms which have also been described as separate species, and even 



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Artewia s.str. 



Callaonella 



Fig. 77. Brine-shrimps (Artemia), from more or less saline water. Upper figures 

 shew tail-segment and tail-fins; lower figures, relative length of cephalothorax 

 and abdomen. After Abonyi. 



referred to a separate genus, Callaonella, closely akin to Branchipus 

 (Fig. 77). Pari passu with these changes, there is a marked change 

 in the relative lengths of the fore and hind portions of the body, 

 that is to say, of the cephalothorax and abdomen: the latter 

 growing relatively longer, the Salter the water. In other words, 

 not only is the rate of growth of the whole animal lessened by the 

 sahne concentration, but the specific rates of growth in the parts 

 of its body are relatively changed. This latter phenomenon lends 

 itself to numerical statement, and Abonyi has shewn that we may 

 construct a very regular curve, by plotting the proportionate length 

 of the creature's abdomen against the salinity, or density, of the 

 water; and the several species of Artemia, with all their other 

 correlated specific characters, are then found to occupy successive, 

 more or less well-defined, and more or less extended, regions of the 



