TWINNING IN EARTHWORMS 31 



{a) those in which the union extends to the alimentary 

 tracts; {h) those in which ahmentary tracts are not 

 united. (2) Those in which the union is latero-dorsal 

 (one example). (3) Those in which union is end to end 

 and in which the cerebral ganglia are double and on op- 

 posite sides of the digestive tract. The union is dorsal, as 

 the nerve cords are on the free (opposite) sides of the 

 united region. (4) Those in which union is lateral, the 

 components lying side by side with the mouth of both on 

 the same side. (5) Those in which the two components 

 are extremely unequal in size. 



It is a significant fact that no cases were found in 

 which the union is ventral. This leads Miss Weber to 

 believe that the single instance cited by Vejdovsky is a 

 misinterpretation and that what he had was a case of 

 dorsal union, the confusion being due to the close 

 approximation of ganglia from opposite sides which lay 

 in the semblance of a paired arrangement. 



As to the causes of twinning in the earthworms 

 Kleinenberg makes the unsupported suggestion that the 

 doubling is due to the entrance of two sperms into a 

 single egg. Such a suggestion is nowadays quite unten- 

 able. Vejdovsky is inclined to adopt a physiological 

 explanation suggesting the possibility that twinning 

 may be due to environmental factors, such as tempera- 

 ture, moisture, or exposure to air. 



Korchelt (1904) described and figured (Fig. 22) an 

 interesting double monster of the earthworm Allohophora 

 suhruhicunda. This is a very typical case and will serve 

 as a type for the whole group. This author also suc- 

 ceeded in producing large numbers of double-headed and 

 double-tailed worms by regeneration methods. After 



