244 INTERNAL ORGANIZATION. 



with very thin sides ; intestines ; a good-sized liver, in 

 the substance of which the said stomach and intestines 

 are enclosed ; branchiae \ a heart, possessing both auricle 

 and ventricle, and surrounding the rectum, on which two 

 great vessels abut, and from which an aortic trunk issues, 

 divided into three branches one for the mouth, another 

 for the supply of the liver and digestive organs, and a 

 third for the remainder of the body. 



The blood of the oyster is colourless. The mollusc 

 adheres to the two valves of its shell by means of a strong 

 muscle (the " adductor ") situated near the middle of its 

 body. 



But though the organization of the oyster is apparently 

 imperfect, it is complete so far as it goes, and possesses at 

 least the indication of organs which, in beings of a more 

 complex type, serve a loftier purpose, and point out the 

 beginnings of Nature, enabling the thoughtful observer to 

 understand her onward course, from the simplest outlines 

 of animal life to the admirable human machine. 



We have spoken of the " mouth " of the oyster ; this is 

 a kind of trunk or snout, with lips remarkable for ten- 

 uity ; it lacks both jaws and teeth. The lungs or gills 

 are foliated coverings extended over the surface of the 

 body, so as to protect the air necessary to the animal's 

 existence from the action of the water. These lamelli- 

 ferm gills are provided with membraneous plates, which 

 act as capillary funnels, open at the furthest extremities. 

 The circulating vessels open into muscular cavities, which 

 play the part of the heart. There is no " foot," locomotion 

 being accomplished by the alternate opening and closure 

 of the shells. The animal secures its food by means of 

 its beard, which acts somewhat after the manner of a rake. 



