THE EARTHWORM 271 



the animal in the dorsal trunk. It is thus forced through the "hearts" 

 and, as it reaches the ventral trunk, is sent both in an anterior and a 

 posterior direction. From the ventral trunk the blood passes to the body 

 wall and nephridia. The lateral neural trunks then receive the blood 

 which has gone to the body-wall, while that having gone to the nephridia 

 has been expelled. The blood in the sub-neural trunk flows posteriorly, 

 then upward through the parietal vessels into the dorsal trunk. The 

 anterior portion of the body receives its nourishment from both dorsal 

 and ventral trunks. 



The Coelomic circulation consists of the fluid in the coelomic cavi- 

 ties. These cavities are continuous throughout all the somites through 

 dorsal apertures or slits occurring between the various septa and the 

 digestive tract. The fluid itself is made up of colorless plasma with 

 white blood cells or leucocytes ( . ). This fluid is 



washed back and forth by the movements of the worm and thus bathes 

 the endothelial lining of the coelom. 



The amoeboid corpuscles in the coelomic fluid have a remarkable 

 power of attacking bacteria and other microscopic organisms such as 

 Gregarines and Infusorians or even small Nematode worms. If such 

 parasites enter the coelom the amoeboid cells surround and destroy them. 

 Their operations are, however, not confined to the inside of the earth- 

 worm. The slime of the body surface is in part composed of mucus 

 secreted by the skin, and in part of coelomic fluid and its corpuscles 

 which find exit through the dorsal pores. The corpuscles are thus able 

 to attack and destroy bacteria before they effect an entry into the body. 

 There is no doubt that a worm is constantly exposed to these minute 

 organisms for the upper layers of the soil teem with them. The slime 

 itself is a protection, for it both arrests the bacteria and holds them 

 stranded in the trail which the worm leaves behind it in its progress. 

 The application of a grain of some irritant, such as corrosive sublimate, 

 enables one to see how a worm protects itself. As soon as the irritant 

 touches the skin the segments in front and behind the seat of injury 

 are forcibly constricted, while the affected segment itself swells up in 

 consequence of the increased pressure brought to bear upon it from both 

 sides. At the same time there is a conspicuous gush of coelomic fluid 

 from the dorsal pores in that region and an abundant secretion of mucus 

 from the skin itself. Thus the threatened region is, as it were, isolated 

 by ligatures from the rest of the body and all the defensive resources 

 at once brought to bear upon the enemy. The coelomic fluid is alkaline 

 and contains crystals of calcium carbonate, and also contains micro- 

 organisms which when isolated and reared in artificial cultures emit the 

 characteristic smell of earthworms. It is, therefore, not improbable that 

 this odor is due to the micro-organisms and not really a feature of the 

 worm itself. 



