450 Dynamic Theory. 



the back of this animal there is a large number as many as ninety- 

 eight have been counted in one individual of regular vertebrate eyes 

 with the inverted retina and consequent blind spot. The arrangement 

 of the lens and other details are less complicated, but all the essential 

 features of a useful eye are present, including rods, pigment layer, &c. 



FIG. 184. Eye on the back of the Onchidium. 



n. Optic Nerve passing through the other layers, 

 p. Pigment. I Lens. 



/.Fibrous layer s. Rods and Cones. 



( After Semper.) 



It is the only example of such an eye in an in- 

 vertebrate animal. Semper has found them in 

 twenty species of this family. They all differ in 

 arrangement, but are normal in structure, which 

 leads him to conclude that they originated in this 

 family. These mollusks live along the sea-shore 

 or in brackish marshes, hiding under stones or in 

 clefts of rock. They are very slow and could 

 hardly get out of the way of anything. They are 

 Fig. 184. pursued by two rapid and active genera of fishes 



called the Periophthalmus. As the Onchidium is out of its shelter 

 scooping sand into its mouth, which is its way of getting at the particles 

 of vegetable matter that forms its diet, this enemy sweeps down upon 

 it and takes it in. Its only defense is from the supposed automatic ac- 

 tion of these eyes. There are throughout the skin of the back of the 

 onchidium a great many small glands which secrete a sort of dense fluid 

 and which, when it is suddenly alarmed by the shadow of its enemy, it 

 is supposed to suddenly contract and throw the fluid upward in small 

 globules through the minute pores which form the outlets of the glands, 

 a performance which may alarm or injure the pursuer in some way. At 

 any rate there seems to be a mutual relationship between these eyes and 

 that fish ; for the mollusk and the fish are found together over a great 

 extent of the eastern coasts of Africa, Asia, and Australia, and the 

 Malayan Archipelago, the mollusk having the dorsal eye's. 



There are many species of the Onchidium without the dorsal eyes or 

 the accompanying glands, living in places not frequented by these 

 Periophthalmi. These are found on the Atlantic coast of France, Eng- 

 land and North America, the Grallapagos Islands, Ne\v Zealand, &c. 

 This active fish is therefore a selective agency in destroying those 

 families of Onchidiums which are not defended by the dorsal eyes, 

 while they can hold their own where he is not. The sudden contraction 

 of the integument by which disagreeable matters are discharged, and 

 which serves for a more or less effectual defense, is characteristic of 

 several families of mollusks, as the Loligo or Squid, the Sea-squirts, 

 Ascidians, &c. 



