VERTEBRA! A. 175 



Gills never present KEFTILIA. 



Warm-blooded ; heart with two auricles and two ven- 

 tricles. 



Oviparous; no mammary glands AVES. 



Viviparous; mammary glands in the female MAMMALIA. 



Class I. PISCES. (Fishes.) 



Vertebrate animals, breathing by gills throughout life, and 

 covered by scales or naked, the scales overlapping each other or 

 imbedded in the skin, or taking the form of detached tubercles or 

 of spines. Heart with one auricle and one ventricle. Blood cold. 

 Limbs in the form of fins ; caudal fin vertical. 



Although the blood is cold, there is no doubt that it is higher 

 than the surrounding temperature, especially in the spawning- 

 season. One of the characters of this class is to have the ver- 

 tebras concave at both ends [amphiccelous] ; the cavity thus 

 formed is filled with the remains of the notochord. The aorta is 

 very generally enlarged at its junction with the ventricle [conus 

 or buibus arteriosus] and is capable of rhythmical contraction. 

 The sound (air-bladder or swiui-bladder), peculiar to fishes, is 

 generally present, and is often connected with the oesophagus by 

 an air-tube. The air in this organ varies according to the species, 

 and is believed to be secreted by the inner membrane ; in some 

 species there are muscles for compressing it. It is mostly simple, 

 but is sometimes cellular, and is variable in form even in species 

 of the same genus. The air-bladder is supposed to regulate the 

 specific gravity of the fish ; but in many good swimmers it is 

 absent, or it may be present in one or absent in a closely allied 

 congener. In Lepidosiren and Ceratod'iis it is lung-like, and acts 

 as a respiratory organ. The skin of fishes is more closely con- 

 nected to the underlying flesh than in other vertebrates. In most 

 there is a lateral line of peculiar scales, each of which is perfo- 

 rated by a tube communicating with a longitudinal canal, giving 

 passage to a mucous secretion produced by the glands beneath 

 and connected with cavities in the head. The use is unknown ; 

 according to Vogt it is a system of absorbent vessels, while Leydig 

 considers it to be subservient to the sense of touch. 



The vertebrae are of two kinds, " characterized by the direction 

 of the parapophyses." They vary in number from 15 to 236, or 

 350 in some of the sharks ; but, owing to the coalescence of some 



