82 The Vertebrate Organ Systems 



give the fish their characteristic slimy feeling. In some fish, there 

 are special poison glands found in association with spines. One of 

 the most highly modified types of glands are those found in deep-sea 

 fish. Some of these form luminous organs known as photophores. 



Among the amphibians, the mucous glands (simple alveolar 

 glands) are the most abundant in the skin. Some tree frogs and 

 salamanders have glandular feet that help them to stick to surfaces, 

 while the toads have rather unpleasant secretions from their skin glands. 



Odoriferous glands are found in a great many different verte- 

 brates. The skunk's odor emanates from a special gland near the 

 base of the tail ; the stinkpot turtle has an odor best described by 

 its name. 



Birds have but few glands over the skin surface. The preening 

 gland mentioned previously is best developed in aquatic birds and is 

 said to have a special odor during the breeding season. 



Pigmentation.— Pigment when present in the skin of vertebrates 

 may be either in the dermis or epidermis. When in the epidermis, it 

 is usually in the form of granules dispersed through the cells. In the 

 dermis, the pigment is contained in special cells known as chromato- 

 phores. These cells are typical of fishes, amphibians, and reptiles. There 

 are three possible types of chromatophores : those containing a dark 

 brown pigment ; those containing yellow or red ; and those containing 

 crystals for light refraction. In man, the different skin colors are due 

 to pigment granules in the epidermis. The pinkish color of the skin 

 is due to the underlying capillaries. 



Color may also be due to physical structures present on scales, 

 hairs, or feathers. There may be fine lines which reflect light. Most 

 feather and hair color is due to the presence of diffuse pigment gran- 

 ules which are derived from pigment cells during development. 



Freckles and moles are due to local concentrations of pigment 

 granules. 



Friction Ridges. — Located on the tips of the fingers are tiny 

 ridges known as friction ridges. These aid the fingers in grasping ob- 

 jects. The sweat glands which open upon these ridges serve to lubri- 

 cate the fingertips, and further aid them in holding tightly. These 

 ridges are due to furrows of the papillary layer of the dermis. The 

 epidermis follows the tiny folds, and thus is thrown into these useful 

 ridges. 



