14 INTRODUCTION. 



transferred to two separate sections of the same organ, though 

 this is still morphologically one. The branchia3 might likewise 

 become internal, as iu Fishes, Crustaceans, Molluscs, Ascidians, 

 and so forth. The current requisite for respiration might, in 

 such a case, be induced simply by the development of cilia on 

 the cells of the epithelium of the branchial membrane, as occurs 

 in all molluscs and in ascidians. The current might then serve 

 another purpose, namely, that of bringing food to the mouth ; 

 and this is the case in the above-mentioned animals, which re- 

 ceive their nutrition, consisting of microscopic organisms, ex- 

 clusively by the aid of the current drawn into the branchial 

 cavity. Now, if the function of respiration were transferred 

 by any means to some other part of the animal, or restricted 

 to a limited section of the branchife themselves, the remaining 

 portion might he transformed into an organ serving exclusively 

 to obtain nutrition. The lips lying near to the mouth of 

 molluscs would, in fact, appear to be such modified portions of 

 the folds of the branchiae. 



But this is by no means the limit of such change of function. 

 Each animal cell in the living organism is sensitive to various 

 molecular movements which imisinge on it from without. 

 General sensibility is an attribute of the living substance of the 

 cell. Now it would obviously be a considerable advantage to 

 the animal that the organs of respiration or locomotion should 

 be connected with certain organs of sense — in our example, for 

 instance, if the lip-like appendages of the branchiae of the 

 molluscs could be transformed into organs of taste or touch. 

 As every living cell, including of course the cells of the mucous 

 membrane of the branchiae or the labial fold, possesses this 

 general sensibility, and this in a certain sense includes the 

 capability for de^'eloping a special sense of touch or taste, we 

 perceive that an epidermal member which originated as a 

 simple gill may, by virtue of its inherent properties, easily be- 

 come an organ of locomotion, sensibility, or taste, and it might 

 eqvially easily be converted into an organ for the acquisition of 

 nouiishment (as in the Ascidians) or for any other purpose. 

 At the same time be it observed, such transformations have 

 not taken place suddenly in an abrupt and, so to speak, revo- 



