12 HIGH SCHOOL ZOOLOGY. 



openings of the olfactory sacs are four in number on tlio dorsal 

 surface of tlic head, but there are no external apertures for the 

 ears. On the ventral surface in front of the anal fin is the 

 posterior aperture of the intestine, and immediately behind it 

 that of the urinary and reproductive organs. On the sides of the 

 head behind the mouth are certain large apertures — the gill- 

 slits, five in numbei', opening into the cavity of the mouth, 

 but these are ordinarily concealed by the gill-cover or opercu- 

 lum, a flap which projects backward over them, and by a 

 membrane attached to the inner surface of the flap, the branch- 

 iostegal membrane. In this way the gill or branchial 

 chamber is formed, opening by the branchial aperture along 

 the hinder and lower border of the said flap. Both the 

 branch iostegal membrane and the gill-cover have a supi)orting 

 framework of bones, the branchiostegal rays in the one case, the 

 opercular bones in the other. 



In addition to the olfactory organs referred to above, the fol- 

 lowing sense organs are to be noted : the eyes ; certain holes 

 and slits along the lateral line and on the head leading into 

 canals and pits in which sense organs are situated ; and the 

 barbels, sensitive processes of the skin of the head, eight in 

 number in the catfish, but frequently absent in other forms. 



5. In most fishes the skin is strengthened by bony scales, 

 either round in outline — cycloid — or with the hinder margin 

 toothed — ctenoid — (Fig. 2), bui the catfish is destitute of 

 such, except for certain very minute ones which are in the 

 walls of the lateral canal. The skin is therefore soft and 

 slip[)ery, and variously coloured according to the distribution 

 of pigment in it. It is tightly bound down to the underlying 

 flesh by slips of fibrous tissue, but in certain parts some loose 

 subcutaneous tissue is accumulated between them. When a 

 sharp cut is made through the skin it is possible to recognize 

 two layers, an outei', the epidermis and an inner, the cerium, 



