364 MANUAL OF ZOOLOGY SECT. 



The geographical distribution of the Cyclostomi is some- 

 what remarkable. Petromyzon is found on the coasts and 

 in the rivers of Europe, North America, Japan, and West 

 Africa. Of the allied genera one, Ichthyomyzon, occurs on 

 the western coast of North America ; another, Mordacia, in 

 Tasmania and Chili ; a third, Geotria, in the rivers of Chili, 

 Australia and New Zealand. Myxine occurs in the North 

 Atlantic and on the Pacific Coast of South America, and 

 Bdellostoma on the coasts of South Africa, New Zealand 

 and Chili. 



CLASS II. PISCES 



The class Pisces or Fishes includes the Elasmobranchii or 

 Cartilaginous Fishes (Sharks, Dog-fishes and Rays), the 

 Teleostomi or Bony Fishes (such as Perch, Pike, Mackerel, 

 Cod, Sole, Salmon, Sturgeon, and Bony Pike), and the Dipnoi 

 or Lung-fishes. In these the organs both of respiration and 

 of locomotion are adapted for an aquatic mode of life. The 

 chief and, in the majority, the only organs of respiration are 

 the gills, which are in the form of series of vascular 

 processes attached to the branchial arches and persisting 

 throughout life. The organs of locomotion are the paired 

 pectoral and pelvic fins, and the unpaired dorsal, ventral 

 and caudal ; these are all supported by fin-rays of dermal 

 origin. A hard external covering of scales developed in 

 the dermis is usually present. In the endoskeleton the 

 notochord is usually replaced more or less completely by 

 cartilaginous or bony vertebrae; there is a well-developed 

 skull and a system of visceral arches, of which the first 

 pair forms upper and lower jaws, the latter movably 

 articulating with the skull, and both nearly always bearing 



