IIO 
COLOUR IN NATURE 
CHAP. 
forms. When fed with food coloured with carmine 
after a previous fast, it was found that the carmine 
could be traced in the excretophores, in the nephridia, 
and in the pigment cells beneath the skin. 
In addition Dr. Graf makes some very ingenious 
suggestions as to the origin of the characteristic 
markings of the different species of leeches. Accord¬ 
ing to him these depend primarily upon the arrange¬ 
ment of the muscles of the body-wall ; or, more 
exactly, the visible coloration depends upon the 
amount of resistance which the tissues offer to the 
passage of the pigment-containing cells, the muscles 
being the most important of the tissues concerned. 
The muscles of the leeches are arranged in three 
layers, which from without inwards are the circular, 
the diagonal, and the longitudinal layers. Each of 
these layers consists of bundles of muscle-fibres, the 
number of the muscles varying in different leeches. 
According to Graf the pigment-containing cells can 
pass outwards only in the spaces between the 
bundles, so that the coloration depends upon the 
number of these bundles, and this varies in the 
different forms. Thus Nephelis quadrostriata has 
five well-developed bundles of longitudinal fibres on 
the dorsal surface, the circular and diagonal muscles 
being less developed, and we find that it has four 
well-marked longitudinal stripes, corresponding to 
the spaces between these muscles. In Clepsine liol- 
lensis, Whitman, on the other hand, the longitudinal 
bundles are very numerous and relatively weak, while 
the circular are well developed ; and here the surface 
is spotted rather than striped, this being supposed 
to be due to the stopping of the pigment cells at 
