190 



THE RISE OF ANIMAL LIFE 



Fig. 10-14. This annelid, called the peacock worm (Fu- 

 distyl'ia), builds its long tube in sand among the 

 rocks. The miniature head, along with many bril- 

 liantly colored gills, protrudes from the end of the 

 tube. The gills function in breathing and food gath- 

 ering, since they are covered with a sticky mucus in 

 which minute sea animals become enmeshed and are 

 then drawn to the mouth by means of cilia. Peacock 

 worms in clusters resemble a bunch of flowers; their 

 real identity becomes apparent when they suddenly 

 withdraw the tentacles into the tube. 



the entire surface of the earth. This is even 

 more surprising when it is known that most 

 of its relatives are aquatic forms. It appar- 

 ently deserted its ancestral watery environ- 

 ment and blazed a trail into a terrestrial 

 habitat of semi-solid soils. In this transition 

 it lost certain of its ancestral parts and ac- 

 quired others. There are few species of 

 earthworms, while there are many species 

 of aquatic forms. 



The industrious robin searching for 

 earthworms is a common sight, but other 

 animals, such as moles, amphibians, small 

 snakes, and fish, also feed upon this form. 

 Some species of earthworms have been 

 found to be intermediate hosts for such 

 parasites as gapeworm and tapeworm of 

 fowls, and lungworms of pigs. Hog influ- 

 enza is caused by the mutual action of a 



virus and a bacterium contracted when 

 lungworms borne by the earthworm invade 

 the respiratory tract. They are highly bene- 

 ficial to man by constantly tunneling the 

 soil, thus permitting a greater circulation of 

 water and air. Charles Darwin noted that 

 their castings on fertile soil amounted to as 

 much as 18 tons per acre per year. This 

 constant elevation of subsoils to the surface 

 tends to cover rocks and gravel, thus mak- 

 ing the topsoil more tillable. In this sense, 

 too, the earthworms benefit man substan- 

 tially. 



External anatomy. The large species, 

 Ltimbriciis terrestris (Fig. 10-15), is a good 

 example of an earthwonu. Its most con- 

 spicuous external characteristic is its seg- 

 mentation, resembling Neanthes in this re- 

 spect. Mere vestiges of the parapods remain 

 in the form of four pairs of very short 

 setae located on each segment. These are 

 used for traction in locomotion and their 

 effectiveness in this capacity is easily ascer- 



Fig. 10-15. The common earthworm (Iwmbricus terres- 

 tris). This specimen had just projected the head end 

 forward and had contracted the posterior half of its 

 body in its typical crawling movements. Note the 

 clitellum. 



