SECT, xnt 



PHYLl'M CHORDATA 



(;•» 



Craniata above 

 i.e., from Ani- 

 np wards, the 



in the iniddlr dorsal line into a vertieal I'uld or incdia/i Jin, whii^h i« 

 continued round the end of the tail and forwards in the middle 

 line to the anus. Frequently this continuous fin becomes broken 

 up into distinct dorsal {d. f. 1 and ?), ventral (v./.), and caudal (cd.f.) 

 fins, which may assume very various forms : in the hif^her cUisses 

 all trace of median tins disap})ears. 



Fishes also possess paired fins. Immediately posterior to the 

 last gill-slit is a more or less horizontal outgrowth, the ijectoral fin 

 {pct.f.), while a similar but smaller structure, the pcfoi'c^ji (l^v.f.), 

 arises at the side of the 

 anus. 



In a 

 Fishes, 

 phibia 



paired fins are replaced 

 by foi-c- and hind-livihs 

 (f.l., h.L), each consist- 

 ing of three divisions — 

 up2^er-arm, fore-arm, and 

 hand in the one case ; 

 thigh, shank, and foot in 

 the other. Both hand 

 and foot normally ter- 

 minate in five fingers or 

 digits, and the 'pentadac- 

 tyle livih thus formed is 

 very characteristic of all 

 the higher Yertebrata. 

 The paired fins, or limbs, 

 as the case may be, are 

 the only lateral appen- 

 dages possessed by Ver- 

 tebrates. 



Body-wall and In- y^ 

 ternal Cavities.— The 

 body is covered extern- 

 ally by a skin consisting 

 of two layers, an outer 

 or epithelial layer, the epidermis (Fig. 758, Ep.), derived from the 

 ectoderm of the embryo, and an iimer or connective-tissue layer, 

 the dermis (Co), of mesodermal origin. The epidermis is always 

 many-layered, the cells of the lower layers, forming the stratum 

 Malpighii, being protoplasmic and capable of active multiplica- 

 tion, while those of the superficial layers often become flattened 

 and horny, and constitute the stratum rorneum. Glands are fre- 

 quently present in the skin in the form of tubular or flask-shaped 

 in-pushings of the epidermis or of isolated gland-cells (B). 



VOL. II F 



;. 7')S. — Diagraiuniutic vertical section of the skin of a 

 Fisb. Ji, uiiicflhilai- mucous glauds ; Co, derm ; E/i. 

 fpiderm ; 1<\ fat ; G, blood-vessels ; Ko, goblet-cells ; 

 K(i, granule-cells ; S, vertical, and /F, horizontal bun- 

 dles of connective-tissue. (From Wiedersheim's 

 Vcitebrata.) 



