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 

 Platiphorella 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 Chitoriidae (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 incisus) ; (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 Tonicia ; 

 and in Acanthopleura, 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 Acanthopleura P air f e y es is developed^at the 



spiniger.' I, optic nerve with ganglionic cells ; c l ose O f larval life, On the SCCOnd 



II, pigmenteil capsule of the eye; III, teg- . .. . _,. 



inentum of the shell ; IV, rods of retina ; shell-valve. I heS6 Organs are 



vh^nrlAfterkoYeiey." 1 ^ ' 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, 



FIG. 33. 



