Phylum Chordata 587 



Tail and Scale Variations. — Among the members of the class 

 Osteichthyes there are several different types of scales. The three prin- 

 cipal kinds are the ganoid, cycloid, and ctenoid (Fig. 22). The ganoid 

 type is found in fish such as the gar ; these scales are somewhat square 

 in shape and have a heavy covering of an enamel-like material, ganoin. 

 The cycloid scales are found among some of the more primitive bony 

 fishes such as the pike; these scales are more or less circular in shape 

 and have concentric rings. The more conspicuous of the rings in some 

 of these forms are indicative of years of growth. The scales occur in 

 overlapping rows. The ctenoid scales such as are found in the perch 

 are very similar to the cycloid scales except that the outer edge has many 

 small spines. 



There are three general types of tails. The hcterocercal such as 

 is found in the sharks has the vertebral column extending into the large 

 upper lobe. The diphycercal is found in the lung fish; in this, the 

 vertebral column extends to the posterior part of the body, and the 

 symmetrical tail is developed below and above the column. The third 

 type of tail, the homocercal, is found in most modern fish such as the 

 perch. In this, the vertebrae extend slightly into the dorsal lobe, but 

 the tail is essentially symmetrical (Fig. 197). 



HOMOCERCAt. 



DIPHYCERCAL HCTEROCERCAL 



Fig. 197. — Types of fish tails. 



Migration of Fish. — Most fish show seasonal movements of a 

 migratory type. Among some it is simply a movement toward shal- 

 lower waters for deposition of eggs, while in others migration is much 

 more extensive. Some species of fish such as the swordfish migrate 

 north in the spring and south in the fall. More spectacular is the fresh- 

 water eel, which makes a trip of thousands of miles from its habitats in 

 the eastern and middlewestern streams to its breeding grounds in the 

 Sargasso Sea near Bermuda. Reversing this process are the salmon 

 which ascend fresh-water streams from their marine habitat for their 

 t^gg deposition. 



