THE INTEGUMENT AND THE EXOSKELETON 49 



placoid scale; the root, or dull lower part, which sets into the jaw and corresponds 

 more or less to the basal plate of the scale; the pulp cavity, the central space, filled 

 in life with a dermal papilla, consisting of connective tissue, blood vessels, nerves, 

 etc. ; the dentine, the bony material composing most of the tooth; the enamel, the 

 thin shiny outer coating of the dentine of the crown. Draw the specimen, 

 coloring ectodermal part blue and mesodermal part red. 



3. The ganoid scale. This type of scale is characteristic of many of the 

 ganoid fishes, such as the gar pike and the sturgeon. Examine a specimen of the 

 gar pike (Lepidosteus) and note its complete investment with hard, shiny, rhom- 

 boid plates, arranged in diagonal rows, fitting closely together. They are typi- 

 cal ganoid scales. Cut out a small piece of skin containing several scales. 

 Note that each diagonal row is movable on the adjacent rows along the line of 

 junction or hinge line, but the members of each row are immovably joined to each 

 other by a peg-and-socket arrangement (visible only on the under side and well 

 developed only in large specimens). Draw a few scales. 



In the sturgeon note similar ganoid scales, bony rhombic plates bearing a short 

 spine. They are arranged in five longitudinal rows, with areas of apparently 

 naked skin between the rows. In some sturgeons (Scaphirhynchus) the ganoid 

 scales form a complete investment for the tail. 



Ganoid scales are composed of bone and are often covered with a shiny substance known as 

 ganoin. They never bear any enamel. They are purely of dermal origin formed by the activity 

 of the mesenchyme cells of the dermis and corresponding to the basal plates of the placoid scales. 

 Primitively they clothed the entire body as in the case of the gar pike, but this arrangement 

 obviously hinders movement; hence to facilitate movement they are often lost from some regions 

 of the body as in the sturgeon, and in the teleost fishes are replaced by thinner, more flexible 

 scales, overlapping each other like shingles. 



4. The cycloid scale. This is the earliest form of the thin, flexible scale 

 and occurs in a few ganoids and some teleosts. Examine the bowfin (Amid), 

 a ganoid fish, and note the thin, rounded scales with which the animal is clothed. 

 These are cycloid scales. They are set in pockets in the skin, and the free pro- 

 jecting edges overlap like shingles, thus allowing greater freedom of movement. 

 Remove a scale and examine under the microscope, or use mounts already pre- 

 pared. Note the smooth border and the markings on the scale. Draw. 



5. The ctenoid scale. This type of scale is similar to the preceding, and 

 occurs in the vast majority of the bony fishes. Note the arrangement of the 

 ctenoid scales on the perch or other common fish. The scales are set in diagonal 

 rows in pockets of the dermis, their free edges overlapping. Remove one or 

 obtain a prepared slide of one and examine with the low power. The attached 

 end of the scale is beautifully fluted; the free border bears several rows of small 

 toothlike projections, which are well developed in the perch but poorly defined 

 in some fishes; and the surface is sculptured with curved parallel ridges. It 



