THE CLASS PISCES. 115 



and tlie siipra-stapedial element of tlie liyoidean arch, are 

 converted into a malleus and an incus, so that, with the 

 stapes, there are, at fewest, three ossicula auditus. 



6. The alimentary canal may, or may not, terminate in a 

 cloaca. When it does not, the rectum opens behind the 

 genito -urinary organs. 



7. The heart is quadrilocular. Some of the blood-cor- 

 puscles are always red and non-nucleated. 



8. There is only one aortic arch, which lies on the left side- 



9. Respiration is never effected by means of branchias, 

 but, after birth, is performed by lungs. 



10. There is a complete diaphragm. 



11. The Wolffian bodies are replaced by permanent 

 kidneys. 



12. The cerebral hemispheres are united by a corpus cal- 

 losum. 



13. The reproductive organs may, or may not, open into 

 a cloaca. The oviduct is a Fallopian tube. 



14. The embryo has an amnion and an allantois. 



15. Mammary glands supply the young with nourishment. 



The Ichthyopsida.— Class I. — Pisces. 



The class of Fishes contains animals which vary so much 

 in their grade of organization, and in their higher forms 

 so closely approach the Ampliibia, that it is difficult to 

 draw up any definition which shall be at once character- 

 istic and diagnostic of them. But they are the only ver- 

 tebrated animals which possess median fins supported by 

 fin-rays ; and in which the limbs, when present, do not 

 exhibit that division into brachium. antebrachium, and 

 manus which is found in aU other Vevtebrata. 



The presence of the peculiar integumentary organs con- 

 stituting what is known as the system of mucous canals and 

 the organs of the lateral line (su^Jra, p. 86), is highly charac- 

 teristic of Fishes, though these organs cannot be said to 

 exist in the entire class. 



The class Pisces is divisible into the following primary groups : — 

 A. The notochord extends to the anterior end of the body. There 

 are no skull, brain, auditory, or renal organs, such as exist in the 



