THE EVIDENCE OF THE AUDITORY APPARATUS 367 



the special sense-organ of this appendage is represented solely by the 

 flabellum. 



This sketch of the special sense-organs of Lirnulus shows that all 

 the appendages of Lirnulus possess special sense-organs, with the 

 exception of the operculum. All these sense-organs are formed on 

 the same plan, in that they possess a fine chitinous tubule passing- 

 through the layers of chitin into the underlying hypodermal and 

 nervous tissues, which terminates on the surface in a pore. The sur- 

 face of the chitin where these pores are situated is perfectly smooth, 

 although, in the case of the branchial sense-organs, the goblet-shaped 

 masses of chitin, each of which contains a pore, are able to be pressed 

 out beyond the level of the surface. 



As to their functions, we unfortunately do not know much that 

 is definite. Patten considers that he has evidence of a gustatory 

 function in the case of the mandibular organs, and suggests also a 

 temperature-sense in the case of some of these organs. The large 

 organ of the flabellum and the branchial organs he has not taken into 

 consideration. The situation of these organs puts the suggestion of 

 any gustatory function, as far as they are concerned, out of the ques- 

 tion ; and I do not think it probable that such large specialized organs 

 would exist only for the estimation of temperature, when one sees 

 how, in the higher animals, the temperature-nerves and the nerves of 

 common sensation are universally distributed over the body. As 

 already stated, the structure of the branchial organs seems to me to 

 point to organs for estimating varying pressures more than anything 

 else, and I am strongly inclined to look upon the whole set of organs 

 as the derivatives of the lateral sense-organs of annelids, such as are 

 described by Eisig in the Capitellidae. This is Patten's opinion with 

 respect to the mandibular organs ; and from what I have shown, 

 these organs cannot be separated in type of structure from those of 

 the flabellum and the branchial sense-organs. 



In our search, then, for the origin of the vertebrate auditory organ 

 in Lirnulus and its allies, we see so far the following indications : — 



1. The auditory organ of the vertebrate is regarded as a special 

 organ belonging to a segmentally arranged set of lateral sense-organs, 

 whose original function was co-ordination and equilibration. 



2. Such a set of segmentally arranged lateral sense-organs is 

 found in annelids in connection with the dorsal cirri of the ventral 

 parapodia. 



