242 



METAZOAN PHYLA 



278. Regeneration. — Earthworms have a considerable power of 

 regeneration and exhibit it in a manner which suggests two axial gradients. 

 One of these has a maximum near the anterior end of the worm and fades 

 out rapidly in the metameres which lie behind (Fig. 151). The other 

 has a maximum at the posterior end and fades out toward the anterior 

 end of the body. A new head can be regenerated at the anterior end of 

 a posterior fragment only as far back as the fifteenth to the eighteenth 

 metamere; beyond the fifteenth the newly formed head is not perfect. 

 A new tail cannot be developed on an anterior fragment farther forward 

 than the twelfth metamere; back of the twelfth a posterior piece may 

 regenerate an anterior tail, but such a two-tailed worm, of course, cannot 

 survive. This existence of two gradients seems to be correlated with 

 the greater sensitiveness of the body at the two ends than in the middle. 



3n 



11 



flffi^ 



Fig. 151. — Diagram to show the possibilities of regeneration in the earthworm. Regen- 

 eration of a head end is perfect back to segment XV, imperfect as far as XVIII; regenera- 

 tion of a tail end occurs back of XII. 



Experiments in grafting earthworms have been tried by suturing 

 fragments together and then permitting them to unite. In this way 

 several pieces may be combined to make a very long worm, a short 

 anterior and a short posterior piece may be united to form a very short 

 worm, two tail pieces may be united to form a worm with a tail at each 

 end, or two tail portions may be grafted to an anterior piece to form a 

 double-tailed worm. Of course such abnormal individuals cannot long 

 survive. 



279. Economic Importance. — Earthworms are exceedingly important 

 economically because of their influence in increasing soil fertility. By 

 opening up the ground and permitting access of air they help to freshen 

 it, and by bringing earth from below to the surface they serve to develop 

 a thicker layer of humus. In passing the soil through their bodies 

 nitrogenous waste is added to it in such form that it can be utilized by 

 plants. The honeycombing of the soil also permits moisture to penetrate 

 it more rapidly. Charles Darwin, in his book on "The Formation of 

 Vegetable Mould through the Action of Worms, with Observations on 

 Their Habits," brought together the results of forty years of observation 

 and a wealth of facts bearing upon this subject. Rarely does the 

 work of earthworms cause injury; the loosening of soil in the walls of 

 irrigation ditches, however, has given trouble in some parts of the West. 



