lymphatic plexus 



pearl organ 



Figure 8-18. Prototriches or "pearl organs" of teleosts. A, section 

 through one on the forehead of Ic/us melonotus; B, on bociy scales of 

 Borbus vulgaris and, C, Phoxinus phoxinus. (After Robl, 193!) 



also guanin containing iridocytes, which are responsible for 

 the silver color of many fishes. The innermost layer of the 

 dermis is densely fibrous. 



is nearly free of fiber; the stratum laxum, a laminated fi- 

 brous layer to which the base of the scale is tied by Sharpey's 

 fibers; the stratum compactum; and a subcutaneous layer 



of variable thickness, in which a reticulum of fine fibers is 

 observed. There is a fairly sharp line of demarcation be- 

 tween the last two. 



The young shark has ciliated cells intermixed with large 

 mucoid cells in the epidermis. As the animal increases in 

 size the cilia are lost and the stratified condition is achieved. 

 Sensory organs (Figure 8-19 C), presumed to be related to 

 prototrichs, are scattered over the body. The scales do not 

 appear till quite late. 



Cyclostomes 



In the lamprey, the epidermis is many cells thick (Figure 

 8-20). The main epidermal cells are very small, and among 

 them are granular and club-shaped secretory cells. The outer 

 layer of this epithelium is modified into "calcareous cells," 

 which have a modified, impregnated, outer surface. These 

 cells are overlaid by a thin cuticle and the entire epidermis 

 is supported by perpendicular fibers from the thin base- 

 ment membrane. 



The dermis is stratified into an outer, thick, laminated, 

 fibrous layer below the epidermis; ne.xt is a thin pigment 

 layer, and below this a reticular subcutaneous layer. There 

 are no scales. 



Myxine is similar in having a thick epidermis covered by 

 mucous (Figure 8-21). The outer cell layer is made up of 

 columnar mucous cells, while deeper in the epidermis are 

 large bladder cells and somewhat smaller thread or filament 

 cells. Along the ventrolateral margins of the body, the 

 epidermis forms saccular slime glands, which utilize the 

 same kinds of secretory cells as the general epidermis. 



The dermis is stratified into an outer layer of fibrous 

 laminae enclosing pigment cells and fibroblasts and a 

 deeper subcutaneous layer of fatty areolar tissue. 



SCALES 



One of the most informative products of the skin, in terms 

 of phylogeny, is the scale. The fish scale is not the homolog 

 of the mammal or reptilian scale that has already been de- 

 scribed. The mammalian and reptilian scales are imbricated, 

 keratinized thickenings of the stratum corneum, while the 

 fish scale is a bony product, primarily of the dermis. The 

 osteoderm of the reptile may be the homolog of the fish 

 scale, or at least the closest approach to it. Teeth are related 

 to, or derived from, true scales. Scales and teeth are easily 

 preserved and present the first fossil evidence of the verte- 

 brates. 



Chondrichthyes 



In the sharks, the points of the placoid scales extend out 

 through the epidermis (Figure 8-19). The dermis has three 

 layers: an outer layer around the necks of the scales, which 



Structural materials of the scales 



By way of introduction, the placoid scale of the shark, 

 which is also presumed to be like the forerunner of the tooth, 



226 



THE SKIN AND ITS DERIVATIVES 



