50 THE AMPHINEURA 
One osphradium is generally present on each side of the internal 
wall of the mantle, near the anus, more or less close to the last 
gill. In the Lepidopleuridae there are branchial sense-organs, 
related to accessory ganglia on the nerve of each gill (Burne). In 
Placiphorella the lower free margin of the mantle bears, in its 
anterior part, several long and thin appendages, which must be 
considered as sensory tentacles (Fig. 32). 
Shell-Eyes.—The tegmentum of the shell-valves is traversed by 
peculiar pallial sense-organs in the form of epithelial papillae, 
containing nerve-endings covered with a cuticular hood. These 
organs are innervated from the pallial cords. According to their 
size, they are termed megalaesthetes or micraesthetes (Moseley). 
In various species of Chitonidae (and specially in exotic littoral 
forms of the sub-families Toniciinae and Liolophurinae), the 
megalaesthetes are converted into eyes, in which retina and pig- 
ment are always present. These eyes are frequently many 
thousands in number, and are most numerous on the anterior 
valve (Fig. 23, I). There are two kinds of such eyes: (1) Extra- 
pigmental eyes, with pigment in the tegmentum only, generally 
without a crystalline lens (except 
Schizochiton inetsus); (2) intra- 
pigmental eyes, with pigment in 
the body of the aesthete, and 
always with a crystalline lens. 
The eyes are arranged in rows 
running diagonally from the 
median anterior beak of the valve 
to its external borders. . There 
may be only one such row on 
either side, as in Schizochiton, or 
many such rows, as in Jonicia ; 
and in Acanthoplewra, in addition 
to the several diagonal rows, 
there are rows of eyes along the 
posterior margin of the valve, but 
these are not present in young 
specimens. In Tonicia the first 
Axial section of a shell-eye of Acanthoplewra pair of eyes is developed, at the 
spiniger. I, optic nerve with ganglionic cells; elose of larval life. on the second 
II, pigmented capsule of the eye; III, teg- 2 
mentum of the shell; IV, rods of retina; Shell-valve. These organs are 
RT at ee comes sensitive to disturbances in the 
water, and do not exist when 
the mucous projections of the mantle, referred to above, are 
present. Even those species of Chitons that are devoid of 
eyes are affected by light and shade: some littoral species, 
such as Boreochiton cinereus, do not move far from the light, 
