EVOLUTION 133 
are ranged in rows running from the front beak of 
each shelly plate diagonally to its outer edge. Some 
species of the pulmonate Oncidiwm have, in addition 
to the eyes on the tentacles, a large number of 
others situated on tubercles over the back. These 
particular Slugs are said only to be found in the 
mangrove swamps where the peculiar fish Perio- 
phthalmus abounds, hopping along the shore in 
search of prey, and it has been suggested that there 
may be some connection between these two occur- 
rences, but nothing very definite has been decided. 
In certain of the Pelecypoda accessory eyes are 
found on the edges of the mantle and at the extrem- 
ities of the siphons. In the Arcide there are com- 
pound eyes to the number of more than two hundred 
placed along the mantle margin. Pecten (Plate XV., 
Fig. 8) and Spondylus have nearly as many, but they 
are isolated and borne each on a short tentacle, more 
being found on the margin of the left, or upper, than 
on that of the right, or lower, mantle lobe. The 
structure of these accessory eyes in Oncidium and 
the Pectenacea resembles the vertebrate eye in that 
the visual rods are turned away from the light. 
Less complex visual organs, but nearly equally sensi- 
tive to light, are found in the mantle margin of 
Ostrea, Pievia, Pinna, and Mactra. The tentacles at 
the ends of the siphons of Cardium (Plate XXIX., 
Fig. 13), Lithodomus, Pholas, Mya, Ensis, and others, 
bear eyes that are more or less sensitive to light and 
