xm 



PHYLUM CHORDATA 



67 



f 



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 



.a Ji a Cf! 



anus. 



In all Craniata above 

 Fishes, i.e., from Am- 

 phibia upwards, t h e 

 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 pentadactyle 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 



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 ; Ko, granule-cells ; S, ver- 

 tical, r and W, (horizontal bundles of connective- 

 tissue. (From Wiedersheim's Vertet/rata.) 



