562 EVOLUTION OF ANIMALS Part V 



that opens externally. At the inner end of the bladder, muscle cells prevent the 

 excreted fluid from flowing back into the body. The essentials of structure in 

 these kidneys are the tubes and their contact with blood capillaries, arrange- 

 ments common to kidneys in general. Some waste substances are taken 

 directly from the coelomic fluid through the ciliated funnel; other by-products 

 are taken from the red blood. The kidney also helps to dispose of excess water. 



Greenish chloragog cells surround the blood vessels and cover the intestine 

 where they are in contact with microscopic capillaries (Fig. 28.9). They take 

 up dissolved wastes from the coelomic fluid and these form the yellowish- 

 green granules within them. When full of such granules they are sloughed off 

 into the coelomic fluid. Some probably disintegrate and their substance passes 

 out through the nephridia; others are taken up by the highly phagocytic 

 ameboid cells. The latter wander into the tissues, disintegrate and their remains 

 are deposited as pigment in the body wall. 



Nervous System — Coordination. The two ganglia that constitute the brain 

 are connected by nerves with another pair beneath the pharynx (Fig. 28.10). 

 From these ganglia the double nerve cord, with a double ganglion in each seg- 

 ment, extends along the ventral floor of the coelom to the last segment. The 

 removal of the brain has little effect upon the responses of an earthworm. How- 

 ever, after the subpharyngeal ganglia are removed, a worm neither burrows 

 nor eats. The neurons in these ventral ganglia are evidently much more im- 

 portant than those in the brain. The ventral cord is the coordinating center of 

 the body. The fibers of sensory neurons extend into it and those of motor 

 neurons out of it as they do in the human dorsal nerve cord. Hundreds of both 

 types of fibers pass through each of its branches (Fig. 28.10). Fibers from the 

 receptor sensory cells connect with the cord. There the impulse on the sensory 

 fiber passes over to an adjustor neuron and thence to a motor neuron which 

 carries the impulse to the effector, in this case a muscle cell. Sensory and motor 

 impulses pass one another on the same nerve but along separate cell fibers as 

 in higher animals. Sensory and motor impulses are continually relayed along 

 the cord over adjustor neurons. 



The waves of muscular action which pass down the body as a worm crawls 

 must be controlled by the ganglia in various parts of the cord since any mod- 

 erate-sized piece of the body will crawl as well as a whole worm. If an earth- 

 worm is touched while outstretched from its burrow, it instantly snaps back, its 

 longitudinal muscles contracting throughout their length. The nervous trans- 

 mission is relatively rapid; it evidently passes over certain giant nerve cell 

 fibers visible when specially prepared sections of the cord are examined 

 microscopically. Experiments have shown that the speed of an impulse over 

 these fibers is 1.5 yards per second. The speed of an impulse over the motor 

 nerve cell fibers in man is about 100 yards per second. 



Reproduction. The reproductive organs are located in the anterior part of 



