II 
THE PIGMENTS OF ORGANISMS 
3i 
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 Liimbricus 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- 
