64 THE EAETHWOEM 



to the mid-dorsal line ; while the outer one is the 

 widest and leads to the external aperture. 



2. Microscopic structure of a nephridium. (Fig. 18.) 



Ca/refully remove an entire nephridium with a very small 

 part of the septum which it traverses. Stain it, dehydrate 

 with alcohol, clear with oil of cloves, and mount in balsam. 



Examine with loio and high powers. 



a. The funnel is ciliated, its border consisting of a row 



of large cells with very distinct nuclei. The open 

 mouth of the funnel can readily be seen. 



b. The first part of the tube is very slender and 



difficult to make out : it lies alongside the inner ' 

 and middle of the loops formed by the wider 

 part of the tube. The canal of the tube is here 

 an intracellular one, traversing the individual 

 cells. 



c. The second part of the tube is much wider, the 



change in diameter being an abrupt one, and the 

 canal having from this point proper cellular walls. 

 It runs up and down the middle loop of the 

 three, at the dorsal end of which it is consider- 

 ably dilated ; then up and down the shorter inner 

 loop ; and finally across to the outer loop, where 

 it becomes continuous with the terminal portion 

 of the tube. The walls are glandular and very 

 vascular. 



d. The terminal portion of the nephridium is much 



wider than the rest : its walls are non-glandular, 

 and composed largely of interlacing muscular 

 fibres. It opens to the exterior by the aperture 

 already seen, in front of the outer seta of the 

 inner double row. 



E. The Eeproductive System. 



The earthworm is hermaphrodite. The genital organs 

 are lodged in segments ix. to xv., i.e. in the region of the 

 oesophagus. 



