90 LABORATORY STUDIES IN ZOOIJKiY 



On the plate of the anterior end of the earthworm designate 

 the parts of the head ; then label the parts of the blood system 

 and digestive tract. On the small drawing in the lower right- 

 hand corner label the space representing coelom and the location 

 of the setae. Write the names on the left of this diagram. 



The Excretory Organs. — The excretory system of the earth- 

 worm consists of a series of paired coiled tubes, the nephridia, one 

 pair of which is present in every segment except the first three 

 and the last. Remove a portion of the alimentary canal caudad 

 of the gizzard by carefully cutting the septa surrounding it and 

 severing it in two places. Then with the aid of a hand lens study 

 the pair of coiled tubular nephridia on the floor of each metamere. 

 The main part of each nephridium consists . of a coiled tube 

 comprising three loops lodged in the coelom of each segment 

 through the ventro-lateral walls of which it opens to the exterior. 

 The aperture is the nephridiopore previously noted in the study 

 of external characters. The coelomic opening of the nephridium 

 is situated in the segment cephalad of the one in which the main 

 structure is located; the tube penetrates the septum and opens 

 into the coelom of the preceding segment by a funnel, called the 

 nephrostome. Seize a septum with the fine forceps and gently 

 move it caudad and cephalad. It will then be possible to 

 recognize on either side of the midventral line the two minute 

 nephrostomes as the septum changes position. 



During life the coelomic cavity is filled with a fluid. The cilia 

 on the nephrostome and in the beginning of the tubule create a 

 current by means of which waste material floating in the coelomic 

 fluid is drawn into the nephridium and in this way eventually 

 conducted to the exterior. The middle portion of each tubule is 

 much thicker and has glandular walls. It functions by removing 

 waste matter in solution from the blood which reaches the walls 

 in a network of capillaries. The portion of each tubule near the 

 excretory pore is dilated and probably subserves the purpose of a 

 bladder. 



On the plate above mentioned label the parts of the excretory 

 system. Write these names on the right of the small drawing 

 at the lower right-hand corner. 



The Nervous System. — In the place where the alimentary 

 canal has been removed identify the vcnlral nerve cord and trace 

 it along the floor of the body wall in the midline. This cord 

 extends throughout the length of the animal. Study its character 



