. Chordate morphology. Morphology (Animals); Chordata. sensory endings' Figure 8-9. Histology of a sensory organ or the scale of a lizard (Agomo). (After Elios and Bortner, 1 957} corneum of soft keratin and a few layers of transitional cells above the flattened (squamous) stratum germinativum (Figure 8-10). The dermis is frequently filled with fat cells. The only glands are the paired multisacculate uropygial glands opening on a papilla above the tail. These glands secrete an oily fluid used by the bird to condition the bill sheath and the feathers. The bill sheath (Figure 8-11) is like the n
. Chordate morphology. Morphology (Animals); Chordata. sensory endings' Figure 8-9. Histology of a sensory organ or the scale of a lizard (Agomo). (After Elios and Bortner, 1 957} corneum of soft keratin and a few layers of transitional cells above the flattened (squamous) stratum germinativum (Figure 8-10). The dermis is frequently filled with fat cells. The only glands are the paired multisacculate uropygial glands opening on a papilla above the tail. These glands secrete an oily fluid used by the bird to condition the bill sheath and the feathers. The bill sheath (Figure 8-11) is like the nail or claw of the mammal; there are also scales and claws on the feet. Claws frequently occur at the tips of the anterior and middle wing digits, at least in the young. The feathers are epidermal derivatives usually described as modified scales. This homology is based on the observation that, in the pigeon, feathers appear to arise out of the scales of the lower leg. The scales of the pigeon leg are really scutes formed by the fusion of several scales. Where scales and feathers are both present, as in the Barn Owl, it is observed that the feather is an interscale structure like the hair. This similarity in scale relationship between feather and hair does not indicate an homology, even though these structures are analogous in that they insulate the body surface. The feathers have two functions: they help in maintaining a high body temperature and they are important for flight. The feathers are of two main types: plumes or pinnaceous structures and plumules or down, including filoplumes. The plumes are distributed on the body in oblique rows and con- centrated into pterylae separated by areas free of such feath- ers, the apteria. Down may cover the apteria and occur among the plumes as well. In structure the feather is com- plex, and in development it stems from a papilla enclosed by a follicle not unlike that from which the hair develops. Amphibians Although it has a thin cellu
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