THE STRUCTURE OF THE OYSTER. 77 



tides the water contains, the latter showing that they 

 tarry on their way, for the outgoing stream is perfectly 

 clear. Here is, in fact, an animal-filtering machine even 

 turbid water becomes clear in the passage ; and if indigo 

 be thrown into clear water, the effect will be strikingly 

 manifest. If, too, the pholas be suddenly taken out of the 

 vessel, the orifices will be closed so that no water can 

 escape, though soon after they will relax, and it will again 

 be spirted out. In this way, then, does the oyster receive 

 and expel water, from vessels similar in their kind, and 

 thus its respiration is absolutely complete. 



Messrs. Alder and Hancock, to whose researches 

 natural history is greatly indebted, have described this 

 beautiful structure as seen under the microscope. Between 

 the gill-plates there are tubes, not unlike those with which 

 we are familiar, as pipes of lead or clay, each consisting of 

 one substance. The walls of the tubes, so to speak, are a 

 regular network, composed of blood-vessels, those passing 

 transversely being the stronger and more prominent. 

 The vessels placed lengthwise, standing a little distance 

 from each other, give to the meshes the form of parallelo- 

 grams figures greater in length than in width. The 

 meshes, moreover, are open spaces, fringed internally with 

 a narrow membrane and active vibratile cilia, which may 

 be described as extremely fine hairs, the name given to 

 them being taken from the Latin word for eye-lashes. 

 Thus the breathing organs of the oyster are nearly the 

 same as the gills of fishes ; they form, in fact, in a kind of 

 network, two double series of vessels on each side the 

 body, imbibing oxygen that pabulum of life and emit- 

 ting the carbonic acid that is generated during the circula- 

 tion of the blood, as completely as these processes occur 

 in the largest animals. 



