4.50 CHORDATA 



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

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

 tltis respect is the character of the skeleton. The cuucular skeleton, the 

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

 remains soft, or contriliutes to a suliordinate degree, more for protection 

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

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

 wdiich, in the lowest vertelirates and the eml)r)-os of the higher, appears as 

 the notochord already mentioned, but in the higher is supplemented b}- the 

 vertebral column and skull. 



The skin of the \-ertel)rates is distinguished from that of all invertebrates 

 bv (figs. ^7, 2S) the many-layered epidermis, and the thickness of the cor- 

 ium. The epidermis is rarelv covered liy a delicate cuticle (fishes, fig. 

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

 land forms, the superficial lavers become cornified aitd hence furnish the 

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

 the deeper stratum ^Ialp)igliii and the superficial stratum corneum (fig. 

 27, sM and 5('; ). 



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

 arises from the mesenchvme. It consists of many lavers of close connect- 

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

 ally the muscles, l.)y a loose tissue rich in Ivmph spaces, the siibculancoiis 

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

 ness, can give rise to protective structures. The hornv laver of the epicier- 

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

 of the turtles, the scales and scutes of die 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 wavs: 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. 500, /). 3. The bonv scales are developed 

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



Hoofs, claws and nails (fig. 50S) 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 ihnc plalc (/>) is the more important. In the mam- 

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

 ■(vhich it extends distally over the upper side of the digit, the claw bed. In 

 claws {im_i;i(cs) the claw plate is curved in both directions, lougitudinallv and 

 transversely (fig. 47,5, HF) reducing the lower cAnc sole (s) . In the hoof (tiiii^iila) 

 the claw plate is cur\-ed transversely (/), the claw sole (s) being reduced to a 



