194 



THE RISE OF ANIMAL LIFE 



l-ubula 



do 



naphrosfomc 



Fig. ]0-18. A schematic side view of one segment of the earthworm showing the details of the 

 nephridium. The nephrostomes have been greatly enlarged. 



system, namely, five principal vessels 

 running the length of the worm, with nu- 

 merous cross-connections and branches 

 supplying blood to all parts of the body. 

 These are the first animals to possess a com- 

 plete circulatory system. All animals above 

 this group also have well developed circu- 

 latory systems. 



The blood of the earthworm contains 

 many white blood cells (leucocytes) float- 

 ing in the fluid plasma. The respiratory 

 pigment is hemoglobin, which is carried 

 free in the plasma, not in corpuscles. The 

 gas exchange takes place where the capil- 

 laries come close to tiie surface of the epi- 

 dermis. For this reason the surface of the 

 animal must be moist at all times, for a dry 

 membrane will not allow the gaseous ex- 

 change to take place. 



The arrangement of the nephridia is 

 similar to that in Neanthes. The nephridium 

 consists of a small ciliated funnel, the 

 nephrostome, which opens into a tiny coiled 

 tubule (Fig. 10-18). This penetrates the 

 septum of the next segment, where it coils, 

 gradually becoming larger and finally ex- 



panding into a bladder-like sac before 

 opening to the outside through the ne- 

 phridiopore. Nephridia are found in all of 

 the segments except the first three and the 

 last. The actual excretory process is carried 

 on by the beating of the cilia of the nephro- 

 stome and the lining of the tubule, which 

 causes waste products from the coelom to 

 enter the tubule and be discharged from 

 the body. Waste materials in the blood are 

 picked up by the glandular portion of the 

 tubule and excreted directly. The chlorogo- 

 gen cells may also aid in this process. 



Behavior. As might be expected, the be- 

 havior and hence the nervous system are 

 more complex in the earthworm than in the 

 lower types. The center of the nervous sys- 

 tem is a bilobed "brain," located in the 

 anterior region dorsal to the digestive tract 

 (Fig. 10-16). The circumpharyngeal con- 

 nectives connect to the ventral nerve cord 

 which consists of a series of ganglia much 

 the same as in Neanthes. There are a few 

 nerve fibers extending into the prostomium, 

 suggesting that this organ is probably sensi- 

 tive to touch. 



