SECT. XIII 



PHYLUM CHORDATA 



69 



in the middle dorsal line into a vertical fold or median fin, which is 

 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.f.), and caudal (cd.f.) 

 fins, which may assume very various forms : in the higher classes 

 all trace of median fins disappears. 



Fishes also possess paired fins. Immediately posterior to the 

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

 (pct.f.), while a similar but smaller structure, the pdvic fin (p0./.), 

 arises at the side of the 

 anus. 



In all Craniata above 

 Fishes, i.e., from Am- 

 phibia upwards, the 

 paired fins are replaced 

 by fore- and hind-limls 

 (f.l., li.l.\ each consist- 

 ing of three divisions 

 upper-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 limb thus formed is 

 very characteristic of all 

 the higher Vertebrata. 

 The paired fins, or limbs, 

 as the case may be, are 

 the only lateral appen- 

 dages possessed by Ver- 

 tebrates. 



Body-wall and In- 

 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 inner 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 corneum. 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 



FIG. 758. Diagrammatic vertical section of the skin of a 

 Fish. B, unicellular mucous glands ; Co, derm ; p. 

 epidei-m ; F. fat ; G, blood-vessels ; Ko, goblet-cells ; 

 A'o, granule-cells ; S, vertical, and W, horizontal bun- 

 dles of connective-tissue. (From Wiedersheim's 

 Vertebrata.) 



