4.30 CHORDATA 



head, trunk, and tail at most six head, neck (cervical), thorax, 

 lumbar, pelvic (sacral), and tail (caudal) occur. Not less important in 

 this respect is the character of the skeleton. The cuticular skeleton, the 

 cause of the annulation of the arthropod, is entirely lacking. The skin 

 remains soft, or contributes to a subordinate degree, more for protection 

 than for support, to the formation of hard parts (dermal skeleton of fishes, 

 alligators, turtles). The firmer tissue is formed in the axis of the body, 

 which, in the lowest vertebrates and the embryos of the higher, appears as 

 the notochord already mentioned, but in the higher is supplemented by the 

 vertebral column and skull. 



The skin of the vertebrates is distinguished from that of all invertebrates 

 by (figs. 27, 28) the many-layered epidermis, and the thickness of the cor- 

 ium. The epidermis is rarely covered by a delicate cuticle (fishes, fig. 

 27, cs); usually such a protection is unnecessary since, especially in the 

 land forms, the superficial layers become cornified and hence furnish the 

 necessary resistance without a cuticle. There are two epidermal layers, 

 the deeper stratum Malpighii and the superficial stratum corneum (fig. 

 27, sM and sc; ). 



The second constituent of the integument, the corium (cutis, derma), 

 arises from the mesenchyme. It consists of many layers of close connect- 

 ive tissue, and is usually separated from the underlying structures, especi- 

 ally the muscles, by a loose tissue rich in lymph spaces, the subcutaneous 

 tissue. Both of these constituents of the skin, aside from their own firm- 

 ness, can give rise to protective structures. The horny layer of the epider- 

 mis in places becomes greatly developed and thus forms the tortoise shell 

 of the turtles, the scales and scutes of the snakes and lizards, the feathers 

 of the birds, the hair and horns of the mammals. Other epidermal prod- 

 ucts are the claws, nails, and hoofs of the terrestrial vertebrates. The 

 corium is often the seat of ossifications which, in contrast to the deeper 

 bones, are called the dermal skeleton. 



The firmness of the vertebrate skin may be increased in three ways: I. Bony 

 scales develop in the corium which project into the epidermis and receive from it 

 a horny outer coat, the horny scale (fig. 509, //). 2. The bony scales are lacking 

 but the horny scales are formed (fig. 509, /). 3. The bony scales are developed 

 but the epidermis remains soft, no horny scales being formed. 



Hoofs, claws and nails (fig. 508) are epidermal structures to be traced back 

 to horny scales, one on the upper, the other on the lower side, enclosing the end 

 of the digit. The first, the claw plate (/>) is the more important. In the mam- 

 mals it grows back more and more into a pocket (w) the root of the plate, from 

 which it extends distally over the upper side of the digit, the claw bed. In 

 claws (imbues) the claw plate is curved in both directions, longitudinally and 

 transversely (fig. 473, ///) reducing the lower claw sole (s). In the hoof (unguld) 

 the claw plate is curved transversely (7), the claw sole (s) being reduced to a 



