34 THE VERTEBRATA OR VERTEBRATES. 



have tubes of sufficient size to allow of the passage of the 

 red particles of the blood. 



Enamel is still denser and harder than dentine ; it is the 

 hardest of all the tissues of the animal structure, and con- 

 tains less animal matter than any other tissue. 



Excepting in a few kinds, such as the halibut, flounder 

 (Fig. 338), and fishes closely related to these, the organs 

 of the Vertebrates are arranged in pairs on the two sides of 

 the body. The eyes are two in number, ears two, and the 

 locomotive appendages never exceed four. And the loco- 

 motive organs are directed towards the heart, and not to- 

 wards the nervous centers as in the Invertebrata. 



All the Vertebrates excepting only the fish known as 

 the amphioxus have red blood which is propelled through- 

 out the system, in veins and arteries, by a muscular organ 

 called the heart. 



The blood of all the Vertebrate animals is composed of 

 a colorless or yellowish liquid called plasma, and exceed- 

 ingly minute particles called corpuscles or disks which float 

 in the plasma. These corpuscles vary greatly in form and 

 size in different animals (Figs. 35-41). 



The corpuscles of the blood of the Vertebrates are of two 

 kinds red corpuscles and colorless corpuscles the former 

 being much more numerous than the latter.* 



* " That the red corpuscles are in some way or other derived from the col- 

 orless corpuscles, may be regarded as certain ; but the steps of the process 

 have not been made out with perfect certainty. There is very great reason, 

 however, for believing that the red corpuscle is simply the nucleus of the 

 colorless corpuscle somewhat enlarged and flattened from side to side ; 

 changed, by development within its interior of a red coloring matter, and set 

 free by the bursting of the sac or wall of the colorless corpuscle. In other 

 words, the red corpuscle is a free nucleus." 



" The origiu of the colorless corpuscles themselves is not certainly deter- 

 mined, but it is highly probable that they are constituent cells of certain 



