194 SEGMENTED WORMS OR ANNELIDA. 



Excretory System, 



As we have mentioned, small particles may pass from the 

 gut to the body cavity, and thence to the exterior by the 

 excretory tubes. There is a pair of these little kidneys, 

 nephridia or segmental organs, in each segment except the 

 first four. Each opens internally into the segment in front 

 of that on which its other end opens to the exterior. They 

 remove little particles from the body cavity, but probably 

 get finer waste products from the associated blood vessels. 

 Nephridia occur in many animals, in most young Vertebrates 

 as well as among Invertebrates, but they are never seen 

 more clearly than in the earthworm. When a nephridium 

 is carefully removed, along with a part of the segment 

 septum through which it passes, and examined under the 

 microscope, the following three parts are to be seen : (a) an 

 internal ciliated funnel, {b) a trebly coiled ciliated tube, at 

 first transparent then glandular and granular, and (c) a 

 muscular duct opening to the exterior. Minute particles 

 swept into the ciliated funnel pass down the ciliated coils of 

 the tube, and out by the muscular part which opens just 

 outside of the ventral bristles. The coiled tube consists in 

 part at least of a series of intracellular cavities, that is to 

 say, it runs through the middle of the cells which compose 

 it ; the external muscular portion arises from an invagina- 

 tion of skin. 



Reproductive System 



The earthworm is hermaphrodite, and its reproductive 

 organs are somewhat difficult to demonstrate with com- 

 pleteness. To see them it will be necessary to dissect 

 several earthworms with special attention to individual parts. 



(a) The Male Organs consist of two pairs of testes, three 

 pairs of seminal vesicles, and a paired vas deferens. 



(i) The testes lie near the nerve cord on the septa 

 between segments lo and ii ; each is "a white translucent 

 body of irregular quadrangular form, rarely more than one- 

 tenth of an inch in diameter." (Fig. 62, T.) 



(2) Mother sperm cells, which give rise by division to 

 young spermatozoa, pass from the testes to the much 

 lobed seminal vesicles, where the spermatozoa are matured. 



