452 ZOOLOGY. 



deoxygenated blood is not completely separated. In 

 Hatter ia and many lizards the pineal gland has the 

 structure of a distinct eye, known as the pineal or parietal 

 eye, situated in the parietal foramen of the skull and 

 covered by a transparent scale. 



CLASS : AVES. 



In the case of Birds one character is sufficient to dis- 

 tinguish them from all other kinds of animals, for they are 

 the only animals which possess feathers, and no bird is 

 without feathers. These structures are peculiar develop- 

 ments of the epidermis, composed of the middle layers of 

 the latter, the outer layers being shed as the feather grows. 

 The feather is developed like a tubular scale or quill, and 

 the tube splits down the posterior side ; the barbs are por- 

 tions of the tube separated by oblique divisions and 

 attached to a longitudinal, undivided thicker portion on 

 the anterior side, which forms the stem or rachis. The 

 lower part of the feather remains tubular and does not 

 shed its outer layers ; this forms the quill, and is sunk in 

 a socket or invagination of the skin. The structure of the 

 wing and that of the leg are peculiar and diagnostic in 

 birds. That of the former is specially adapted for flight, 

 the requisite extent of surface being supplied not by the 

 limb itself but by the feathers. The radius and ulna are 

 parallel to each other, not crossed, the radius being 

 anterior and smaller than the ulna. The carpal bones are 

 reduced to two, there are three metacarpals united together, 

 of which the first is very short and bears a single phalanx, 

 the second bears two phalanges, the third one. The 

 peculiarity of the leg is that the tarsus is entirely ankylosed 

 with the neighbouring bones, and the ankle joint, instead 

 of being between the tibia and the tarsus, is between the 

 proximal and distal rows of tarsals ; the proximal row is 

 united with the tibia, and the distal with the second, third, 

 and fourth metatarsals, which are united together. The 

 first metatarsal is free, but the fifth is absent. The fibula 

 is vestigial. The segments of the leg are therefore the 

 femur, the tibio-tarsus, the tarso-metatarsus, and lastly the 

 digits. There are no teeth, the jaws being enclosed in a 



