MOLLTJSCA : THEIE EYES. 67 



possess. You see on each tentacle a little wart, wliich 

 when you look at it with a lens you perceive to have 

 a round black glossy extremity. This is the eye. By 

 careful dissection under the microscope, we find it to 

 contain a beautiful transparent crystalline lens, with a 

 thick and glutinous viti"eous humour adhering to it be- 

 hind, bounded by a retina or curtain to receive the op- 

 tic image, and an optic nerve. 



But much more attractive you will find the eyes in 

 this little Scallop. It is a half-grown individual of 

 what is provincially known as the Squin {Peoten oper- 

 cularis), much prized for its delicate sapidity. Belong- 

 ing to the bivalve class of the MoUusca, the animal is 

 inclosed within two shallow shelly plates, concave in- 

 ternally, and convex externally, which are united by a 

 hinge, just as the works of a watch are protected by 

 the case. When the little creature is at its ease, as 

 when the water is pure and clear, it lies on one side, 

 its valves being separated as we see them now, a quar- 

 ter of an inch or so apart, allowing us to discern what 

 is contained between them. 



Well, we see first a number of slendez-, white-pointed 

 threads, peeping out from each valve and spreading on 

 all sides, waving hither and thither, groping, now con- 

 tracting, now expanding, with incessant but deliberate 

 motion. These are tentacles. If we trace them to 

 their origin, we find them attached to a fleshy sort of 

 veil that lines each valve to near its edge, and then ab- 

 ruptly falls at an angle towards the opposite valve, 

 where it meets a corresponding veil. These two veils 

 form the mantle. It is from each of these that the ten- 

 tacles spring ; and we discover that there are four rows 



