CHAPTER XXI. 



Class Pisces. Fishes. 

 General Survey. 



By the exclusion of the lancelet and the Cyclostomata, the 

 class of Pisces has been made more homogeneous. 

 There are four orders : — 



(4) Bony or Modern fishes, — Teleostei, such as Cod, 



Flounder, Salmon, Herring, Eel. 

 (3) Ganoid fishes, — Ganoidei, such as Sturgeon I^Aci- 



penser) and Bony-Pike {Lepidosteus). 

 (2) Mud-fishes or Double Breathers, — Dipnoi, Cera- 



todus and Protopterus. 

 (i) Cartilaginous fishes or Elasmobranchii, such as 

 Skates and Sharks. 

 With the Elasmobranchs may be ranked the Holocephali, 

 including Chimmra and Callorhynchus. 



General Characters. 



Fishes are aquatic Vertebrates, breathing by gills the 

 oxygen dissolved in the water. In Dipnoi a single or double 

 outgrowth from the gut — the air- or swim-bladder — is used as 

 a lung. The same structure is present in Ganoids and in most 

 Teleosteans, but usually serves solely as a hydrostatic organ, 

 though sometimes of some slight usefulness in respiration. 



Two pairs of limbs are almost always present, as paired 

 fins supported by fin-rays. Fingers and toes begin with 

 Amphibians. The unpaired fins which fringe the body are 

 also supported by rays. 



There are two great types of paired fins. In Dipnoi and 



