THE CLASS PISCES. 103 



14. The embryo has an amnion and 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 Amphibia, that it is difficult to draw up 

 any definition which shall be at once characteristic and diag- 

 nostic of them. But they are the only vertebrated 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 all 

 other Yertebrata. 



The presence of the peculiar integmentary organs con- 

 stituting what is known as the system of mucous canals and 

 the organs of the lateral line {supra, p. 79 ), is highly charac- 

 teristic of Fishes, though these organs cannot be said to exist 

 m the entire class. 



The class Pisces is divisible into the following primary groups : 

 A. The notoohord extends to the anterior end of the body. There are no 

 skull, brain, auditory, or renal organs, such as exist in the higher Yerte- 

 brata. The heart is a simple tube, and the liver is saccular. (Lepio- 

 CAKDiA. Haeokel.) 



I. — Pharyngobranchii. 

 B The notoohord ends behind the pituitary fossa. A skull, brain, auditory, 

 and renal organs are developed. The heart is divided into auricular and 

 ventricular chambers. The liver has the ordinary structure. (Paciiy- 

 CARDIA. Hck.) 



a. The nasal sac is single, and has a median external aperture. Neither 



mandibles nor limb arches are developed. (Monorhina, Hck.) 



II. — Marsipobranchii. 



b. There are two nasal sacs with separate apertures. Mandibles and 



limb arches are developed. {Amphirhina. Hck.) 



a. The nasal passages do not communicate with the cavity of the 



mouth. There are no lungs, and the heart has but one auricle, 

 a. The skull is devoid of membrane bones. 



III. — ElasmobranchU. 

 p. Membrane bones are developed in relation with the skull. 



1. The optic nerves fosm a chiasma, and there are several 



rows of valves in the aortic bulb. 



IV. — Ganoidei. 



2. The optic nerves simply cross, and there is only one 



row of valves in the aortic bulb. 



Y.—Teleostei. 



b. The nasal passages communicate with the oral cavity. Tlier* 



are lungs, and the heart has two auricles. 

 VI. — Dipnou 



