36 THE PHYSIOLOGY OF TWINNING 



parts are separate and the dorsal united. This is just 

 the opposite of what we find in the vertebrates, but is 

 precisely what we would expect in view of the fact that 

 the nervous system is dorsal in the vertebrates and 

 ventral in the annelids. 



CAUSES OF TWINNING IN THE OLIGOCHAETA 



The method of reproduction in the Oligochaeta is 

 the well-known one of cocoon formation which, at the 

 risk of repeating what every biologist knows, may be 

 stated briefly as follows: During copulation a tough 

 girdle composed of hardened slime is formed about the 

 cHtellum of each worm. After the pair separates, each 

 sHme girdle which is destined to be a cocoon is slowly 

 worked forward, collecting albumen from the glands 

 on the ventral surface. It is sloughed off over the 

 head, passing first the openings of the oviducts where 

 it receives eggs, then that of the seminal receptacles 

 where it receives sperm. The two free ends close as 

 though with a drawstring and the closed cocoon is 

 dropped on the ground or in the mud. It has already 

 been pointed out that, at least in some species, some of 

 the eggs die and decay. This would greatly foul the 

 contents of the cocoon. Whether some eggs die or not, 

 the Hving eggs must develop with a limited supply of 

 oxygen. It is this condition, I believe, that is at the 

 bottom of the twinning process. Lack of oxygen 

 probably so retards early development at a time when 

 abundant oxygen is demanded that the bilateral pri- 

 mordia become physiologically more or less completely 

 isolated. Isolation occurs more extensively at the points 

 of highest rate of metaboHsm, the anterior and posterior 



