THE MICROSCOPE. 77 



the two. Oystermen call the thin end the nib, and the thick end 

 the bill or beak. I now hold this oyster in my hands edgewise, so 

 that the flat valve is next my right hand, and the round valve next 

 my left hand. The nib faces me, and the beaks are from me. The 

 flat shell is the right side and the round shell is the left side. The 

 under edge is the ventral, and the upper edge the dorsal part. The 

 populai notion is that the thin part of the shell, the place where the 

 knife enters when opening it, is the forward end of the oyster. This 

 is not so. All will admit that the head end of any animal should 

 be regarded as its front, or anterior end. Now, the cephalic portion 

 of an oyster lies here near the hinge; hence, you see the hinge end, 

 or beak of an oyster, is its anterior part, and the nib, or thin or 

 opposite end, is the posterior part. 



Everyone knows that each for himself possesses a threefold 

 physiological system, that is, three sets of organs specialized for 

 three sets of functions. These are, the nutrient, the respiratory and 

 the circulatory systems, so-called. These, as their names imply, 

 have to do respectively with alimentation or food elaboration, with 

 the aeration or oxygenation of the vital fluids, and the circulation of 

 the blood. Let us see if these systems are possessed by an oyster, 

 and to what extent they are developed in a creature so lowly. 



2. Alimentation. — An oyster is enveloped in a thin, loose mem- 

 brane, called its pallium, or mantle. This pallium is united for a 

 little way at the left side of the animal, and all round the right side, 

 but at the posterior end it is free. Let us look at this large diagram. 

 I have torn off the right side of the cloak, thus exposing the animal, 

 which, with the flat shell removed, is represented as lying in the 

 round or dished shell. This leaves the prominent organs exposed. 

 The edge of the pallium is beset with rather coarse fleshy hairs, 

 which are cilia. As there are two edges, they make a double fringe. 

 When the shell is a little bit open this ciliated fringe is slightly pro- 

 jected, and is called the beard. The two fringes keep up an inces- 

 sant vibrating action which sets the water flowing in between them; 

 and another contrivance, soon to be described, carries the food-laden 

 water forward over the body, or ventral side of the animal, up to 

 this spot near the hinge, where it is received on the edges of four 

 flattish lips, the mouth being between the two pairs. We may call 

 these organs palpi, or labial fingers, for they assort this food from 

 the water, which done, the water thus filtered is passed backward on 



