u THE PIGMENTS OF ORGANISMS 31 



and the pale muscles of Vertebrates the difference 

 of colour is usually associated with differences in 

 chemical composition and histological character, and 

 in certain cases at least with physiological differences 

 in relation to the reaction to stimuli. It is there- 

 fore sometimes supposed that the contained haemo- 

 globin is of respiratory importance, and is the cause 

 of the physiological characters. On the other hand, 

 in certain cases as in the rabbit, the association 

 between the colour and the other characters is not 

 very close, and it is to be noted that insects also 

 possess two kinds of muscle distinguished inter alia 

 by their colour ; in their case the extensive develop- 

 ment of the tracheal system forbids the idea that the 

 difference is caused by the presence or absence of a 

 special respiratory pigment. 



In Invertebrates the difficulties connected with the 

 supposition that haemoglobin is always of supreme 

 importance as a respiratory pigment are even greater. 



It is said that haemoglobin is especially necessary 

 to Lumbricus on account of its peculiar habitat ; that 

 its presence in the head-slits of Nemerteans is 

 essential for the oxidation of the brain ; that it is 

 present in the muscles of the buccal mass of Littorina 

 because these muscles are especially active ; and so 

 on. On the other hand, many large marine worms 

 have no haemoglobin, whatever their habitat, and 

 many Gasteropods have none in their buccal muscles. 

 Can we suppose that these muscles are less active in 

 the limpet, the snail, and many others than in 

 Littorina ? 



The more these questions are considered, the 

 more difficult does it become to suppose that haemo- 



