258 ANIMAL CANNONEERS AND SHARP-SHOOTERS. 



or head eyes of Onchidimn differ in no ways from those of alUed 

 groups, but its dorsal eyes (and it commonly has from twelve to 

 sixty, one species having even as large a number as eighty, 

 according to Lubbock ; another species, according to Semper, 

 has ninety-eight) are identical, as far as type is concerned, with 

 those of vertebrate animals. These dorsal eyes have corneae, 

 retinae, and lenses, anterior and posterior chambers, and "blind- 

 spots." The " blind-spot " is peculiarly characteristic of the 

 vertebrate eye : the optic nerve pierces the external layer of the 

 retina ; hence at this point sight is absent, 



Now. of what use are these twelve, sixty, or ninety-eight eyes 

 in the back of this creature, staring up, as they do, in all direc- 

 tions ? They must subserve some useful purpose, otherwise they 

 would not be present ; and they do, as I shall now endeavour to 

 show. 



Wherever you find the Onchidiimi^ you will be certain to 

 observe likewise a very peculiar fish whose family name is Perioph- 

 ihalmus. This fish has the habit of leaving the water and coming 

 out on shore, where it seeks its food, being enabled by its long 

 ventral fins to make its way over the sands very rapidly in succes- 

 sive leaps, and Onchidimn is its favourite food. The coriaceous 

 back of this mollusc contains a multitude of glands which secrete 

 a thick, tenacious substance — almost a concretion, in fact. In 

 some preserved specimens that I examined not long ago, the con- 

 tents of these glands were concretions, resembling minute shot. 

 The preserving fluid, however, may have been instrumental in 

 hardening the contents of the glands. The integumental pores of 

 these glands are exceedingly small. Now, when Periophthalmus 

 comes leaping over the sands, bounding several inches into the air 

 at each leap, the staring dorsal eyes of Onchidium catch sight of 

 the enemy. Immediately the mollusc contracts the coriaceous 

 skin of its back and discharges thousands of viscous pellets from 

 its dorsal glands at its foe. Periophthalmus, now alarmed and 

 dismayed (overwhelmed, as it were, by this shower of shot from a 

 masked battery), turns and flees for its life, and the watchful 

 Onchidium is saved from a deplorable fate. Periophthalmus itself 

 is a very uncanny-looking creature, with its pair of great staring 

 eyes situated in the top of its head. As it leaps along the sea- 



