370 SEA-SIDE STUDIES. 



a glimpse of a nervous system, than we at once conclude it to 

 be similar in structure with our own. In lilce manner. Con- 

 tractility is supposed to imply the presence of a muscular 

 system ; and the muscles of the lowest animals are supposed 

 to be only minuter forms of the same organs found in man. 

 But direct study of Nature assures us that there may be 

 Sensibility without nerves, and Contractility without mus- 

 cles ; it further assures us that the nervous and muscular 

 structures, when present, differ greatly in different forms of 

 the animal series ; and that we must understand these dif- 

 ferences if we would understand Physiology. The law of 

 unity of composition, when rightly understood, is a guide of 

 great value ; but it is too often a will-o'-wisp, luminously 

 misleading. We get into the habit of naming organs 

 according to a general idea of their functions, instead of 

 keeping the idea of special function dependent on special 

 organ constantly before us. What would be said of a man 

 who called his gig a gondola, his steamer a railway, his 

 carriage a wheelbarrow ? The general function of all these 

 things is the same, but the special office of each differs from 

 that of the others, and its structure bears reference to such 

 specialty. In like manner a " proboscis " is not a " hand," 

 although both are prehensile organs ; nor are contractile 

 fibres " muscles," although both subserve the same general 

 function. 



Descending from these abstract considerations to the 

 particular subject now before us, namely, the nervous 

 system, we note first, that minute and laborious as the 

 researches have been, they have seldom taken the direction 



