XIII 



PHYLUM CHORDATA 



67 



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 2), ventral (v.f.), and caudal (cd.f.) 

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

 all trace of median fins disappears (cf., however, the Cetacea). 



Fishes also possess paired fins. Immediately posterior to the 

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

 (pet. f.), while a similar but smaller structure, the pelvic fin (pv.f.), 

 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-limbs 

 (f.L. h.L), each consisting 

 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 terminate 

 in five ringers or digits, 

 and the pentadactyk limb 

 thus formed is very char- 

 acteristic of all the higher 

 Vertebrata. The paired 

 fins, or limbs, as the case 

 may be } are the only 

 lateral appendages pos- 

 sessed by Vertebrates. 



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. 767, Ep.), derived from the ectoderm of the em- 

 bryo, 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 multiplication, while those of 

 the superficial layers often become flattened and horny, and con- 

 stitute the stratum corneum. Glands are frequently 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 muscular layer. This is always 



Co 



FIG. 767. Diagrammatic vertical section of the skin of 

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

 CS, cuticular margin ; Ep, epiderm ; F, fat ; G, blood- 

 vessels ; Ko, goblet-cells ; K6, granule-cells ; S, ver- 

 tical, 'and W, 'horizontal bundles of connective- 

 tissue. (From Wiedersheim's Vertebrata.) 



