44 ABIOLOGYOFCRUSTACEA 



globin concentration in Artemia. The salinity is clearly an indirect 

 factor, because it acts by decreasing the amount of oxygen that the 

 water can hold. 



Artemia differs from Daphnia in that the females make more 

 haemoglobin per unit body size than the males. Another difference 

 is that Artemia does not pass haemoglobin into its eggs. It does 

 however produce resting eggs (p. 72) which have hard brown shells, 

 and it has been found that these shells are very rich in a haem 

 compound similar to the haemochromogen in the gut of Daphnia. 

 It should be mentioned that Artemia also has a haemochromogen 

 in the gut and thus differs from the notostracan, Triops cancri- 

 ] or mis, which also produces eggs with shells rich in haemo- 

 chromogen, but does not have such a pigment in its gut. 



The blood pigments of the Crustacea are not contained in cells, 

 they are dissolved in the plasma. The only cells in the blood are 

 those which correspond with the white cells in mammals. In many 

 Crustacea there appears to be only one type of blood cell. Artemia, 

 for instance, has blood cells which vary in diameter from 10-15/x; 

 they are normally more or less spherical, but they often adhere to 

 various organs and creep about in an amoeboid manner. Daphnia 

 has similar, but smaller blood cells 7-8// in diameter. Crabs and 

 lobsters have several types of blood cells which might be progressive 

 stages in the development of a single type. The blood cells are 

 formed in special organs which occur at the bases of the limbs (in 

 Artemia), on the wall of the stomach (in the crayfish), in the head 

 (in Amphipoda) or on the surface of the ventral artery (in Stomato- 

 poda). Each organ is formed from a group of cells surrounded by a 

 thin membrane. The cells within the membrane all look alike, but 

 show various stages in division. The newly formed blood cells 

 presumably escape through the thin membrane into the blood. 



The blood cells perform various functions. They are active in 

 resisting infection by bacteria and fungi. Metchnikoff (1884) first 

 gave a detailed account of this process and realised its importance 

 in resistance to diseases; he found a population of Daphnia magna 

 which was infected with a yeast-like fungus which produces long, 

 needle-like spores. These spores penetrate the gut of Daphnia and 

 enter the blood, where they germinate and reproduce. Eventually 

 the whole body-cavity may be filled with spores, but a high percent- 

 age of the individuals which become infected manage to combat the 

 infection. The blood cells gather around the spores and destroy 

 them. 



