ANATOMY AND LIFE HISTORY OF MOLLUSCA. 129 



through shell substance. In the centre is a small dark circular 

 spot with a brilliant speck of light reflected by the lens. 



It is better to explain on the threshold of these observations, 

 a difficulty which will occur to most persons not very intimately 

 acquainted with the subject, as to how these eyes on the outside 

 of the shell could communicate their impressions to the nerve 

 centres of the animal. This difficulty will be met most easily in 

 the words of Mr. Mosely, remarking at the same time that the 

 shell, no matter how hard or apparently independent of the 

 animal, is a structure intimately associated with the integument, 

 and is always permeated more or less by nerve-channels between 

 the different plates of shelly matter. Now in the case of 

 the Chitons, this is the more easily understood from their 

 peculiar structure. They were formerly termed multivalve shells ; 

 because they are covered with eight moveable plates of shelly 

 matter, sustained in their places by a cartilaginous frame which 

 is a horny extension of the mantle. The imbedded portion of 

 each plate is produced into two processes belonging to the lowest 

 plate of shelly matter. The exposed portion is very much 

 thickened, of a rugose sculpture and colored, forming a kind of 

 raised, triangular, winged area at each side. This is called the 

 tegmentum, and at the junction of these two plates of shell are 

 the openings through which the nerve branches are given off to 

 form the optic nerves as described hereafter. I may mention, 

 that though Mr. Mosely regards this structure as peculiar to the 

 Chitonid?e, something similar to it exists amongst a very large 

 number of Mollusca, both univalves and bivalves, as far as my 

 observation goes ; though of course modified according to the 

 peculiar structure of each. It is more apparent in the case of 

 Chitons, but it is hard to understand in many cases how the 

 juncture is effected, yet I think I shall be able to show how 

 the' result is attained in those species in which I have found 

 eyes. I now give the result of Prof. Mosely 's examination. 



"The entire substance of the tegmentum in the Chitonidae, is 

 traversed by a series of branching canals, which are occupied, in 

 the living condition of the animal, by corresponding ramifications 

 of soft tissues, accompanied by abundance of nerves. The nerves 

 and strands of other soft tissue enter the substance of the 

 tegmentum along the line of junction of its margin with the upper 

 surface of the articulamentum. A narrow area, perforated all 

 over by pores, so as to have a sieve-like appearance, here intervenes 

 between the two components of the shells, and in some shells the 

 actual margin of the tegmentum itself is perforated. In the case 

 of the intermediate shells, in most genera there are a pair of slits 

 (incisurce later ales), one on either side, in the lateral lamina of 

 insertion ; these slits lead to two narrow tracts in the deeper 



I— July 4, 1888. 



