ORIENTATION 



otreta). They are parasitic on other fishes in the adult. The notochord and 

 its surrounding sheaths serve as the main skeletal axis. True vertebral elements 

 do not reinforce the notochord, although certain vertebral elements are present 

 in some species. 



Examples: The California hagfish, PoUstotrema (Bdellostoma) stoiiti, and 

 the common sea lampreys, Petromyzon marinus, Okkelbergia lamotteni, Lam- 

 pet ra ayresii. 



The California hagfish has 12 pairs of gill slits whereas the sea lamprey, 

 Petromyzon marinus, has 7 pairs. 



Group II: Gnathostomata 



The Gnathostomata are vertebrates which possess jaws. In a sense, they 

 are the only true vertebrates in the chordate phylum, for the notochordal axis 

 always is supplemented or displaced by vertebral elements. 



1. Class: Pisces 



Division 1 : Chondrichthyes 



To this group belong the selachian or elasmobranch fishes. The word chon- 

 drichthyes means cartilaginous fishes, i.e. the fishes with endoskeletons of 

 cartilage. The adjective selachian has a similar meaning, whereas the term 

 elasmobranch means plate-like gill. 



The sharks, skates, rays, and chimaeras comprise the numerous species of 

 cartilaginous fishes. The skin is covered with small placoid scales; median and 

 paired fins are present; the sexes are separate, and elaborate reproductive ducts 

 are developed. The heart, exclusive of the sinus venosus, is two chambered. 



Examples: Sqiialus acanthias, the dog fish; Rhineodon typus, the whale 

 shark; Manta birostus, the "great devil ray." 



Division 2: Dipnoi 



The dipnoan or lungfishes effect external respiration by means of gills and 

 well-formed lungs. The heart, in harmony with its respiratory mechanisms, 

 is practically three-chambered. Paired fins have a segmented, cartilaginous, 

 central axis. 



Examples: The African lungfish, Protopterus annectens; the South Ameri- 

 can lungfish, Lepidosiren paradoxa; and the Australian lungfish, Neoceratodus 

 forsteri. 



Division 3: Teleostomi 



In this group, the skeleton, in most species, is bony. A single opening for 

 the gill-chamber is present -on each side of the pharynx, the gills being cov- 



