no 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 hol- 

 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 



