io COMPARATIVE ANATOMY 



epithelium, or if it is an internal surface an endothelium, of 

 which more hereafter. Finally, there is the reproductive 

 system. The animal casts off a minute and formless part of 

 its substance, and this part under appropriate conditions 

 grows, assumes the shape and structure of the whole form 

 which it was cast off, and becomes a new animal, like to its 

 parent. Reproduction is a complex process which will be 

 better understood at a later stage. 



An animal, then, of a degree of organisation comparable to 

 our own, has muscular, skeletal, digestive, glandular, blood- 

 vascular, respiratory, excretory, nervous, sensory, and repro- 

 ductive organs, each performing its special function, and all 

 working together to the ends of the individual. The detailed 

 study of the functions of the animal body, and the exposition 

 of the mechanisms and chemical actions by which those 

 functions are performed belongs to the science of animal 

 physiology. We are only concerned with the materials of 

 which the organs are constructed, what are their form and 

 composition, how they are combined, and how the organs are 

 fashioned and put together in different kinds of animals. In 

 short, the comparative anatomist is concerned with the archi- 

 tecture of the building, and gives but a passing heed to the 

 uses to which the parts of the building are put. 



Comparative anatomy, then, is the science which treats of the 

 architecture of animals, and we shall see that just as there are 

 several styles of architecture, Classical, Byzantine, Gothic, etc., 

 each with its subdivisions, as Gothic into Pointed, Perpen- 

 dicular, Decorated, etc., so there are several styles of animal 

 architecture, and in each style there is infinite variety, though 

 the general plan, the " motive," is the same. 



But even as all buildings resemble one another in the most 

 general way because, whatever their style, they are put to 

 similar uses, so do all animals resemble one another in a very 

 general way because, being animals, they have similar functions 

 to perform, and those functions are performed by analogous 

 organs. 



Not by any means, however, by organs which have any 

 degree of exact similitude. Let us compare two or three 

 familiar kinds of animals with one another ; a dog, a fish, and 

 a lobster. The fish (say, a cod or pike) and the lobster are 

 both aquatic animals, and it might be supposed that they 



