392 THE MICROSCOPE. 



excite the greatest surprise when they are pointed out to us. The 

 truth of this observation is forcibly exemplified as regards fish. We 

 see them every day exposed for sale on stalls, and we eat them fre- 

 quently at our tables, without once considering by what a curious and 

 delicate organisation these creatures are enabled to see and breathe in 

 an element that carries death to us and to quadrupeds. The sight of 

 fishes appears to be remarkably strong, as it is by sight chiefly that 

 they discover their prey. Hence a fish is easily deceived by an arti- 

 ficial fly, or the imitation of a frog or other small aquatic or amphi- 

 bious animal ; which, if it were guided by the smell, or any other 

 sense than the sight, could not happen. The mode in which fishes 

 breathe is, however, the most curious. They have no lungs ; but, 

 instead of them, they have gills, carefully covered with a lid and a 

 flap, both of which the fish can open or keep closed at pleasure. The 

 gills are composed of arches bordered by a kind of fringe, which, when 

 examined through the microscope, is seen to be covered with a velvet- 

 like membrane, over which myriads of wonderfully minute blood-ves- 

 sels are spread, like a delicate network. There are commonly four of 

 these fringed arches : they are movable, and allow the currents of 

 water driven down by the action of the mouth to flow freely through 

 them, so as to lave every fibril. It is absolutely necessary that this 

 should be the case, since the gills lose their power of acting as soon as 

 they become dry; and hence a fish cannot live long after it is taken 

 out of the water. As there is danger, however, of the food taken by 

 the fish being carried through the gills by the stream of water con- 

 stantly flowing through them, the minor curve of the arch formed by 

 the gills is studded with spines, which prevent any thing but air or 

 water passing through them. 



A knowledge of the form and structure of scales of fishes, like that 

 of teeth, has been shown by M. Agassiz to afford an unerring indication 

 of the particular class to which the fish may belong : in the examination 

 of fossil remains, the application of this knowledge has been attended 

 with extraordinary results. As a class of objects for the microscope, 

 the scales of fishes are exceedingly curious and beautiful, especially 

 when mounted in fluid or Canada balsam, and viewed by polarised 

 light. Many are seen best as opaque objects, and are then mounted 

 dry between glasses. M. Agassiz divided the scale into four orders, 

 which he named Placoid, Ganoid, Ctenoid, and Cycloid ; in the first 

 two the scales are more or less coated with enamel, in the others they 

 are of a horny nature. To the Placoid order belong the skates, dog- 

 fish, ray, and sharks ; cartilaginous fishes, having skins covered with 



