8 BATEACHIANS. 



Still more rarely it is armed with spines." Passing over the 

 general economy of fishes we come to the heart. " The heart," 

 he tells us, "" consists of one auricle receiving the venous blood, 

 and one ventricle propelling it to the gills or organs submitting 

 that blood in a state of minute subdivisions to the action of 

 aerated water. From the gills the aerated blood is carried over 

 the entire body by vessels, the circulation being aided by the 

 contraction of the surrounding muscles." 



The functions of gills are described by the Professor with great 

 minuteness. " The main purpose of the gills of fishes," he says, 

 " being to expose the venous blood in this state of minute sub- 

 division to streams of water, the branchial arteries rapidly divide 

 and sub-divide until they resolve themselves into microscopic 

 capillaries, constituting a network in one plane or layer, supported 

 by an elastic plate, covered by a tesselated and non-ciliated epithe- 

 lium. This covering and the tunics of the capillaries are so thin as 

 to allow chemical interchange and decomposition to take place 

 between the carbonated blood and the oxygenated water. The 

 requisite extent of the respiratory field of capillaries is gained by 

 various modes of multiplying the surface within a limited space." 

 " Each pair of processes," he adds, " has its flat side turned towards 

 contiguous pairs, and the two processes of each pair stand edgeway 

 to each other, being commonly united for a greater or less extent 

 from their base; hence Cuvier describes each pair as a single 

 bifurcated plate, or ' feuillet.' ' 



The modification which takes place in the respiratory and 

 other organs in Eeptilia, is described in a few words. " Many 

 fishes have a bladder of air between the digestive canal and the 

 kidneys, which in some communicate with an air-duct and the 

 gullet ; but its office is chiefly hydrostatic. When on the rise of 

 structure this air-bladder begins to assume the vascular and 

 pharyngeal relations with the form and cellular structure of 

 lungs, the limbs acquire the character of feet : at first thread- 

 like and many jointed, as in the Lepidosiren ; then bifurcate, or 

 two-fingered, with the elbow and wrist joints of land animals, 

 as in Amphiuma ; next, three-fingered, as in Proteus, or four- 

 fingered, but reduced to the pectoral pair, as in Siren." 



In all reptiles the blood is conveyed from the ventricular part 



