352 APPLIED BIOLOGY 



(4) Excretory Organs. In each segment, except a few near the 

 mouth, there are two delicate coiled thread-like tubes which connect 

 with pores on the sides of the body between the two rows of bristles. 

 The inner ends of the tubes are funnel-like and open into the body- 

 cavity. Minute blood-vessels in the walls of these tubes supply 

 them with blood, from which the cells of the tubes take nitrogenous 

 excretions. From time to time a small amount of fluid from the 

 body-cavity is allowed to flow through the tubes and wash the ex- 

 cretions to the exterior. These tubes act, then, as simple kidneys ; 

 and the several hundred pairs of such kidney-tubes (nephridia) in 

 a large worm co-operate in doing the work such as the frog's kidneys 

 (each containing hundreds of tubes) do for that animal. 



Carbon dioxide is excreted by absorption from the skin, and some 

 also in the fluid ejected by the kidney-tubes. 



(5) Nervous System. The main nerve-cord is on the ventral sur- 

 face, and is easily seen in a specimen from which the digestive organs 

 have been carefully removed. In each segment there is a thickened 

 portion, known as a ganglion, and containing many nerve-cells. 

 Two pairs of nerves in each segment run from the main nerve-cord 

 to the various organs in that region. Dorsal to the anterior end 

 of the pharynx lie two ganglia (cerebral ganglia), sometimes errone- 

 ously called the "brain," and from them two small nerves extend 

 around the esophagus to the main nerve-cord on the ventral side. 

 Note that the main nervous organs of the earthworm are on the 

 ventral side of the digestive canal, while in the frog (and all verte- 

 brates) they are dorsal. 



303. Development of Earthworm. Although both ova 

 and sperm-cells are produced by each individual, self-fer- 

 tilization is guarded against as effectually as in many flowers 

 we have studied. In each worm there are in segment 10 

 some small sacs or sperm-reservoirs which are filled with 

 sperm-cells from the spermaries of another individual when 

 two worms pair in early spring. These sperms are stored 

 until time for laying eggs. The eggs are deposited in a case 

 or cocoon formed as follows. A secretion is poured out on 

 the surface of the girdle (clitellum) of the worm, this secretion 

 hardens to form a ring, and then between the ring and the body 

 a jelly-like nutrient substance is secreted. The ring is then 

 worked forward by contraction of the worm's body, and, 



