CHAPTER XIV 



PIGMENT TISSUES 



ANIMALS, in certain of their tissues and products of these tissues, are 

 variously colored. These colors depend upon refractive surfaces, dif- 

 fracting lines or markings, and inclosed colored substances called pig- 

 ments. The various substances giving rise to the colors of the tissues 

 and of their products are included under the term pigments. These 

 pigments may be inert, serving merely as coloring and protective devices ; 

 or may be of great physiological importance. They occur either in a 

 diffused or in a segregated form. Considered as coloring substances, 

 they serve to protect from light or to conceal from the observation of 

 enemies and prey and to attract or to warn other organisms. 



Examples of diffused pigments among animal products are the 

 bilirubin of the bile, the urochrome of the mammalian urine, the purple 

 secretion of several nudibranch mollusks, and the brown or black secre- 

 tion of the cephalopod mollusks. Other varieties, based upon their 

 physiological use, might be demanded by physiologists in a complete 

 classification, but the above must suffice as examples of what we mean 

 by diffused pigments in tissue products. In general we shall consider 

 here only such pigments as apparently exist to serve the animal by their 

 color. 



Many organisms have colors peculiar to them, which are due to dif- 

 fused pigments in the tissues themselves. Among the Protozoa the 

 members of the genus Vampyrella often present a diffused red pigment, 

 which gives the specimens a characteristic color. Many of the colors 

 of the insects are due to diffused pigments. Myochrome is a diffused 

 pigment that gives the red color to mammalian muscle. Perhaps the 

 most important diffused pigment is the haemoglobin in the blood of 

 all vertebrates and some invertebrates; in this case the pigment is a 

 substance, the physiology of which we understand and in which the 

 color is, possibly, more of an incident than a point of any importance 

 to the economy of the organism. It can be said, however, that the color 

 of blood does serve a distinct end. Other such diffused pigments of 

 various colors are found in the blood of invertebrates. In many of 



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