PRESENCE OF EYES TN SHELLS OF CERTAIN CHITONIDA3. 41 
difference between the two in size being considerable, and 
there being no pores of intermediate size between the two. 
The mouth of each megalopore leads into a cylindrical 
chamber hollowed out in the thickness of the tegmentum, per- 
pendicular to its surface and more or less dilated in accord- 
ance with the form of the papilliform body contained within 
it. This cylindrical chamber is continued below into a wide 
canal, which in its course towards the plane of junction of the 
tegmentum with the articulamentum is curved towards the 
girdle margin of the tegmentum (PI. VI, fig. 6, pp). On 
reaching the plane of junction it joins a plexus of wide main 
canals which ramify horizontally in this plane, parallel with 
the surface of the tegmentum. 
From the sides of the megalopore chambers are given off 
fine canals, which perforate the tegmentum in a direction ver- 
tical to its surface, and join the bases of the micropore cavities. 
In some species a considerable proportion of the micropore 
canals are also given off direct from the main vertical branches 
of the horizontal plexus, as in Corephium aculeatum (see 
PI. Y, fig. 8). Those springing from the megalopores may be 
given off from each macropore chamber at the same, or nearly 
the same, height all round, or at very various heights (see PI. 
IV, fig. 10, bb). 
The tegmentum when decalcified persists as a homogeneous 
apparently horny substance, which in some species shows a 
finely fibrous structure (PI. VI, fig. 4), but in others appears 
almost structureless. This substance, which is in the recent 
state of the shell impregnated with the lime salts, is termed by 
Middendorf the stroma, and by Marshall, Reincke and Van 
Bemmelen the cuticula. It retains in the decalcified condi- 
tion both the form and dimensions of the tegmentum itself, 
and thus in sections of the decalcified shell the disposition of 
the contained soft structures with regard to the hard parts is 
clearly displayed. 
The plexus of horizontal main canals is occupied in the 
horny shell by a corresponding ramification of strings of soft 
tissue, which are offsets of the mantle substance. These 
