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 Oncidium 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 Arcidae 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, Pteria, Pinna, and Mactra. The tentacles at 

 the ends of the siphons of Cardium (Plate XXIX., 

 Fig. 13), Lithodomus, Pholas, Mya, Busts, and others, 

 bear eyes that are more or less sensitive to light and 



