76 



EARTH WOliMS, LKKCIIKS, A>,'1) SEA WORMS 



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

 tissue extends vertirall.v through the body at every constric- 

 tion and (h\'ides the l)ody iuternaUy into as many compart- 

 ments as there ai'e segments. 



Bilateral symmetry. — If, when th.e earthworm is in a 

 natural pdsiliim with the dorsal side uppermost, the body 

 is cut lengthwise, exactlj' on the middle line, into tw(^ etjual 

 hah'es, the right half will be exactly similar to the left half. 

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

 sijuninini." It is a sti'ucture common to the higher animals 

 and reaches its gi-eatest ijerfection in the hunian body. 



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

 woi'm is made ii|) of two tubes, a small one witliin a con- 



Braincjanglion ^}orlic arches- 



Ganglia '■ Oviducf "' -^"^'(f'tci/ onjans 

 Ovary 



Fk). 3!i. — niagiaiii of tlic internal ^traoture of an (■artlu\()rin. 



sideraldy lai'gei' one. The small, innei- tube is the alimen- 

 tary canal (ipening in front fiy tlie mouth and liehind by 

 the amis. The outer tube is foriued l)y the body walls of 

 the woi'm ;nid since it is so much larger than the imiei' tube 

 thei'c is a, space suriounding the aliiuentary canal but in- 

 cl')sed l)y llie body walls. This sp)ace is known as the body 

 cavity, oi' ai'luiiic. Here again we hnd a structui-e com- 

 mon to the higliei- animals. Within tlu' liody of a. spai'row 

 or labliit is a sjjace surrouiuliug the alimentary canal Init 



