STRUCTURE OF FISHES. 583 



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

 the fish being carried through the gills by the stream of 

 water constantly flowing through them, the minor curve 

 of the arch formed by the gills is studded with spines, 

 which arrangement prevents everything but air or water 

 passing through them. For viewing the circulation of the 

 blood, take the young Stickleback. 1 Fig. 324. 



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

 (fig. 323) 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 re- 

 sults. As a class of objects for 

 the microscopes, 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, Fi - 323 -~ Scale f Solc - 

 Dog-fish, Ray, and Sharks ; cartilaginous fishes, having skins 

 covered with small prickly or flattened spines. To the Ganoid 

 belong the Sturgeon, Lepidosteus, Hassar-fish, and Polyp- 

 terus ; fish of this order are more generally found in a fossil 

 state, and their scales are of a bony structure. To the Ctenoid 

 belong the Pike, Perch, Pope, Basse, Weaver-fish, &c. ; their 

 scales are notched like the teeth of a comb. To the Cycloid 



(1) A remarkable observation, with regard to the Stickleback, is deserving of 

 record. This active little favourite of the Aquarium bears, without incon- 

 venience, to be transferred from his native fresh-water pond, to the Salt-water 

 companionship of Actinia, &c., and appears equally at home and happy in the 

 denser medium. 



