CH. l] EARTHWORMS AND LEECHES 9 



Dogfish, and other Elasmobranch fishes, and the somewhat 

 similar structure in the intestine of the medicinal leech 

 and again the radial " mesenteries " of Sea- anemones and 

 their allies are mechanisms adapted to securing the same 

 end as the typhlosole, viz. that a tube whose internal 

 surface shall greatly exceed its external in superficial 

 extent. 



Respiration and Circulation. In the absence of special 

 organs of respiration this function is in the worm per- 

 formed by the outer surface of the body. In . each 

 segment of the body a pair of blood vessels is given 

 off from the subintestinal longitudinal trunk to the body- 

 wall and skin, where the interchange of oxygen and carbon 

 dioxide is effected through the moist surface of the inte- 

 gument. The oxygenated blood is then returned either 

 to the subneural trunk or to the vessels which run in 

 the walls of the anterior portions of the digestive system, 

 the blood being kept in motion by the pairs of contractile 

 " hearts " in segments 6 or 7 11. These hearts run from 

 the longitudinal dorsal vessel round the oesophagus to the 

 subintestinal vessel. Astonishment is sometimes expressed 

 that worms can breathe underground. But the soil is 

 seldom so closely packed that there is not a fair supply 

 of air entangled in the spaces between the particles of 

 earth, and doubtless the aeration of the soil is sub- 

 stantially enhanced by the very burrowing of the worms 

 themselves. At the same time it is noteworthy that 

 their blood is provided with a special vehicle of oxygen 

 hemoglobin not indeed contained in corpuscles as in 

 the blood of vertebrate animals, but dissolved in the 



