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 Limulus shows that all 
the appendages of Limulus 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 Capitellide. 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 Limulus 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, 
