62 



ZOOLOGY 



SECT. 



urino-genital aperture, are sometimes found either in front of or 

 behind the anus, but more commonly the urinary and genital ducts 

 open into the termination of the enteric canal, or cloaca, so that 

 there is only a single egestive opening, known as the cloacal 

 aperture. On either side of this there may be a small abdominal 

 pore (ah. p.) leading into the ccelome. 



In Fishes and some Amphibians, the trunk and tail are produced 

 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 (c.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 pelvic fin (pv.f.), 

 arises at the side of the 

 anus. In the embryonic 

 E8 condition there is some- 

 times found to be a low 

 ridge (r.) connecting the 

 pectoral and pelvic fins 

 of each side with one 

 b another, and from this 

 and other considerations 

 there is reason for think- 

 ing that the paired fins 

 are detached and en- 

 larged portions of a 

 continuous lateral fin, 

 having similar anatomi- 

 cal relations to the meta- 

 >pleural folds of Amphi- 

 oxus. 



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 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 terminate in five fingers or digits, and 

 the pvn&adaetyle limb thus formed is very characteristic of all the 



Co 



FIG. 713. Diagrammatic vertical section of the skin of a 

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

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

 Ko, granule-cells ; S, vertical, and W, horizontal bun- 

 dles of connective tissue. (From Wiedersheim's 

 Fertebrata.) 



