514 CHORD ATA. 



whioli, like the ambulaeral system, opens to the exterior. Two deep-sea 

 forms, Cephalodisciis and liliahdopleitra, liave the same type of ' noto- 

 cliord,' and the first has a pair of gill slits. In other respects these are 

 strikingly Polyzoau in appearance. 



SuK Phylum I^'. A'ertebkata. 



In the vertebrates only the internal segmentation occurs. This 

 is shown, and most clearly, in the lower Vertebrata, in the muscles 

 (myotomes, m3'omeres), the myocommata or myosepta which sep- 

 arate them, and the protovertebra3 from which they arise; in the 

 nerves (neurotomes), the skeleton (sclerotomes), the blood-vessels, 

 and in the excretory organs (nephrotomes). In the higher verte- 

 brates this metamerism is visible only in the embryonic stages. 

 In part this absence of external segmentation has its cause in the 

 heteronomy (p. 390) of the body and the obliteration of segmental 

 boundaries, consequent upon the union of somites into body re- 

 gions, of which at least three — 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 charac- 

 ter of the skeleton. The cuticular skeleton, which is the cause of 

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

 remains soft, or contributes to a subordinate degree, more for pro- 

 tection than for supjiort, to the formation of a skeleton (dermal 

 skeleton of fishes, alligators, turtles). On this account firmer 

 tissue is formed in the axis of the body, which, in the lowest ver- 

 tebrates and the embryos of the higher, appears as the notochord 

 already mentioned, but in the higher is supplemented by the verte- 

 bral column and skull. 



The skin of the vertebrates is distinguished from that of all 

 invertebrates by two characters (figs. 26, 27) : the manv-layered 

 condition of the epidermis, and the considerable thickness of the 

 derma. The epidermis is but rarely covered by a delicate cuticle: 

 usually such a protection is unnecessary since — aud especially in 

 the hind forms — the superficial layers become cornified and hence 

 furnish tiie necessary resistance without a cuticle. There are two 

 layers to be distinguished, the deeper stratum ^Malpighii ami the 

 superficial stratum corneum (,sJ/aud ,<r; see p. 7G). 



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

 corium), arises from the mesoderm (mesenchyme). It consists of 

 many layers, often stratified, of close connective tissue, and is 

 usually separated from the underlying structures, especially the 

 mirscles, by a loose tissue rich in lymph spaces, the subcutaneous 



