40 ABIOLOGYOFCRUSTACEA 



activity of the species. For instance, in the sluggish stone crah 

 (Lithodes) the concentration of magnesium is much higher than in 

 the active prawns. Various other species can he arranged in a series 

 between these two according to their activity and the concentration 

 of magnesium in their blood. 



Another important variable in the composition of the blood is the 

 presence or absence of a respiratory pigment capable of combining 

 loosely with oxygen and distributing it to the tissues. Two such 

 pigments are found in Crustacea : haemocyanin and haemoglobin, 

 but never together in one species. 



The Decapoda and Stomatopoda have haemocyanin in the blood. 

 This is a compound containing copper and a protein. It combines 

 with oxygen in proportion to its copper content; two atoms of 

 copper to one molecule of oxygen. When combined with oxygen 

 it is blue, but when not so combined it is colourless. The molecular 

 weight of haemocyanin is very high. If the weight combining with 

 one molecule of oxygen is considered the molecular weight can 

 be put at 74,000, but if the size of the particles in solution in the 

 blood is considered then the figure is much higher; in the lobster 

 it is about 640,000. In the prawn Pandalus and the crawfish 

 Palinnrus the particles in the blood have a molecular weight of 

 360,000. The crayfish, Potamobias, and the crabs Cancer and 

 Carcinus have particles in the blood with both the molecular 

 weights given above. 



Although these molecular weights are large they do not compare 

 with the haemocyanins of some molluscs, where a molecular weight 

 of over six million has been recorded for the respiratory pigment 

 of a snail. These large molecules are not just an accident, but appear 

 to play a definite role in maintaining the osmotic concentration of 

 the blood at a level similar to that of sea water. If the haemocyanin 

 was dispersed in smaller more numerous particles the osmotic 

 pressure of the blood would rise, and the animal would be con- 

 fronted with a greater osmotic intake of water. 



Haemoglobin has been found in the blood of the Anostraca, 

 Notostraca, Conchostraca, Cladocera. some parasitic copepods, some 

 harpacticid copepods, some ostracods, the branchiuran Dolops, and 

 some of the parasitic Cirripedia. 



The haemoglobin in Cladocera, particularly Daphnia, has been 

 intensively studied by Prof. H. Munro Fox and his school. The 

 blood of Daphnia is often bright red with haemoglobin, but the 

 colour shows surprising fluctuations. These are due mainly to 



