128 



THE RATE OF GROWTH 



[CH. 



described as the specific characters of A. milhausenii, and of a 

 still more extreme form, A. kop'peniana ; 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 referred to a separate genus, Callaonella, 

 closely akin to Branchipus (Fig. 33). 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 



^ w w 



Fig. 33. 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.) 



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 lately 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 curve (Fig. 33). In short, the 

 density of the water is so clearly a "function" of the specific 



