xii PHYLUM CHORDATA 365 



cod, consuming the flesh in its passage, and thus becomes 

 for a time an internal parasite — the only example among 

 the Vertebrata of such a condition. In the free state Myxine 

 usually lies buried in the sand, with only the anterior end, 

 with the nasal aperture, projecting on the surface. By 

 means of the passage leading from the nasal sac to the 

 mouth, water passes in and out through the nasal aperture, 

 and the process of respiration is carried on while the ani- 

 mal remains almost completely hidden. 



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, includ- 

 ing the Straits of Magellan ; 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, dogfishes, and rays), the Teleo- 

 stomi 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 the pelvic fins, and the unpaired dorsal, 



