XIII 



PHYLUM CHORDA TA 



63 



higher Vertebrata. The paired fins or limbs, as the case may be, 

 are the only lateral appendages possessed by Vertebrates. 



Body-wall and Internal Cavities. --The body is covered 

 externally by a skin consisting of two layers, an outer or epithelial 

 layer, the epidermis (Fig. 713, JEp.\ derived from the ectoderm of 

 the embryo, and an inner or con- 

 nective-tissue layer, the dermis 

 (Co), of mesodermal origin. The 

 epidermis is always many-layered; 

 the cells of the lower layers, 

 forming the stratum Malpigliii, 

 being protoplasmic and capable 

 of active multiplication, while 

 those of the superficial layers 

 often become flattened and horny, 

 and constitute the stratum cor- 

 ncurii. Glands are often present 

 in the skin in the form of tubular 

 or flask-shaped in-pushings of 

 the epidermis or of isolated gland- 

 cells ( B ). 



Beneath the skin comes the 

 musrular layer. This is always 

 highly developed, and, in the 

 lower Craniata, has the same 

 general arrangement as in Am- 

 phioxus, i.e. consists of zig-zag 

 muscle-segments or myomeres 

 (Fig. 714, mym.), separated from 

 one another by partitions of con- 

 nective tissue, or myocommas 

 (myc.), and formed of longitudin- 

 ally disposed muscle-fibres. The 

 myomeres are not placed at right 

 angles to the long axis of the 

 body, but are directed from the 



t 



.sagittal plane outwards and back- 

 wards, and are at the same time 

 convex: in front and concave be- 

 hind, so as to have a cone-in- 

 cone arrangement (Fig. 715, C). 

 Each mvomere, moreover, is 



t, 



divisible into a dorsal (d. m.) and a ventral (v. m.) portion. In 

 the higher groups this segmental arrangement, though present in 

 the embryo, is lost in the adult, the myomeres becoming converted 

 into more or less longitudinal bands, having an extremely complex 

 arrangement. 



