MOLLUSCA, THE SHELL-FISH in 



along the floor or table in the direction of the 

 stripes; then try moving it in an opposite direc- 

 tion across the stripes. It is easy to perceive 

 that in the former case one's eye does not detect 

 the movement nearly so soon as in the latter 

 case. To explain this would necessitate a lengthy 

 digression on the subject of optical illusions : 

 /that the fact is so everyone may easily ascertain 

 by experiment. The ridges, therefore, converging 

 in the direction towards which the shell is going, 

 are a protective decoration, enabling it to slip 

 away more easily from under the eyes of its foes. 

 The reader will readily recall a parallel instance 

 in the common Cockle. This also is a very 

 active creature ; it takes leaps by means of a 

 strong muscular foot ; and the ridges on the 

 shell, like those of the Scallop, converge towards 

 the hinges, that is to say, in the direction in 

 which the shell moves. Another instance of a 

 very active shell-fish with similar markings is 

 afforded by certain kinds of Lima, a near relative 

 of the Scallops. It may be added that all 

 Scallops are not equally active, nor all Limas ; 

 and various modifications of their form and 

 colour might be pointed out which lead us to 

 suspect that in the less active kinds the pattern 

 of ridges is often somewhat obscured by means 

 of these differences. 



Now, take up a comb and draw it over your 

 fingers, firstly along the teeth, and secondly across 

 them, and you will be able to estimate the gain 

 in speed and comfort to the comb-shell, Pecten, 

 and to the common Cockle, from having its 

 ridges set in the direction in which it is going. 

 Were the ridges concentric, as is so often the 

 case in bivalve shell-fish of a more sluggish dis- 



