76 



EARTHWORMS, LEECHES, AND SEA WORMS 



ficial wrinkles of the skin, but, in reality, a partition of thin 

 tissue extends vertically through the body at every constric- 

 tion and divides the body internally into as many compart- 

 ments as there are segments. 



Bilateral symmetry. — If, when the earthworm is in a 

 natural position with the dorsal side uppermost, the body 

 is cut lengthwise, exactly on the middle line, into two equal 

 halves, the right half will be exactly similar to the left half. 

 " This similarity is called two-sided likeness, or bilateral 

 symmetry." It is a structure common to the higher animals 

 and reaches its greatest perfection in the human body. 



The general plan of the body. — The body of the earth- 

 worm is made up of two tubes, a small one within a con- 



BrqingangUan Aortic arches- 



Dmsm 0{fo,o^ vessel 



OuUeb Crop Gaza. 



^^^j^4} ^^^^^ ^ r^n '' W!i^^ - 



Ganglia ' lOvickidt^^^^iif^ei/ organs 

 Ovary 



Fig. 39. — Diagram of the internal structure of an earthworm. 



siderably larger one. The small, inner tube is the alimen- 

 tary canal opening in front by the mouth and behind by 

 the anus. The outer tube is formed by the body walls of 

 the worm and since it is so much larger than the inner tube 

 there is a space surrounding the alimentary canal but in- 

 closed by the body walls. This space is known as the body 

 cavity, or caelome. Here again we find a structure com- 

 mon to the higher animals. Within the body of a sparrow 

 or rabbit is a space surrounding the alimentary canal but 



