58 NATURAL HISTORY. 



sion which reigns throughout, we discover, on reaching 

 the reptiles, that their heart has lost one ventricle, and 

 their blood is cold ; and in fishes we find it has lost an 

 auricle, and consists only of a single auricle, and a single 

 ventricle. In the fishes, also, there are striking changes 

 in the composition of bone ; the skeleton gradually loses 

 its hardness, until we come to the Myxines, where it is 

 soft and membraneous, or, it might be said, "the internal 

 hard frame called the skeleton has disappeared, and we 

 are brought step by step to the consideration of soft ani- 

 mals without vertebrae ; but, nevertheless, they are pro- 

 vided with organs suitable for maintaining the kind of 

 life with which they are endowed by the Omniscient 

 Creator of all things." This concludes our account of 

 the vertebrate animals which constitute the First Branch 

 of the Animal Kingdom. We will now consider the 

 Second Branch, composed of invertebrate animals, at 

 the head of which stands the Mollusca. 



