CHEMISTRY OF THE SEA 167 



Kaiser Wilhelm Canal, at 17.3%c salinity; farther to the west, with 

 10.6/co, it is 54.4 mm., and finally at 3-5% salinity it grows to only 

 38 mm. 3 Heincke compares two flounders (Pleuronectes platessa) 

 21 cm. in length from the North Sea and the Baltic; the former was 

 three years old, the latter six. 



Among the most persistent of the inhabitants of the brackish waters 

 of the Baltic and of the Mediterranean lagoons is the annelid Nereis 

 diversicolor, which is also able to resist highly concentrated saline 

 waters. The small number of stenohaline brackish-water animals, con- 

 fined to brackish waters, includes the hydroid polyp, Cordylophora 

 lacustris, and crustaceans such as Eurytemora hirundo and Temora 

 longicomis of the Baltic. Among fishes the Syngnathidae are inclined 

 to take to brackish water. No group of animals has produced as many 

 brackish-water forms as the mollusks. Whole genera of lamellibranchs 

 and snails have gone over to this habitat. The greater number of 

 brackish-water forms are at present tropical, where the immigration 

 from the sea into this habitat and into fresh water appears to be much 

 easier than in higher latitudes (cf . p. 360) ; the reasons for this are still 

 unknown. The production of special brackish-water forms may occur 

 in two ways: by mutation, so that the direct relation with their marine 

 ancestor is no longer evident; or by the extinction of the ancestral form 

 in the marine habitat, so that the brackish-water derivative remains 

 as a relict. The latter seems to be plainly the case with the clam 

 Tagelus gibbus, now confined to brackish waters in America, while 

 its marine ancestors have been found fossil in the Miocene of North 

 America and the Argentine. 4 



Calcium compounds. — Among the more important constituents of 

 the sea salts, calcium is the only one whose proportionate amount 

 suffers important fluctuations. It varies from 1.59 to 1.82% of the total 

 salts, averaging 1.6%. Since the sea water is in general alkaline, it 

 does not retain calcium carbonate in solution, and the calcium occurs 

 principally as sulphate, in contrast with the condition in fresh water, 

 which is able to dissolve the carbonate, thanks to the presence of free 

 carbon dioxide. The calcium is utilized by innumerable animals in 

 building up spicules, shells, armor, and skeletons. The CaS0 4 is usu- 

 ally precipitated for this purpose by (NH 4 ) 2 C03. Experiments show 

 that this reaction proceeds very rapidly at 26-29°, but very slowly 

 at lower temperatures (4-7°). 5 The free use of calcium salts by 

 marine animals is thus conditioned to a considerable extent by tem- 

 perature. It has been shown by direct observation that crabs change 

 their shells and grow new ones more rapidly at high temperatures than 

 at low. Calcium deposits by animal life reach their maximum in the 



