48 THE LEECH. 



slide in a drop of water, and examine with low and high 



poivers. 



The entire nephridium consists of the following 



parts. 



a. The testis-lobe forms the ventral half of the anterior 



limb of the loop. It is a cylindrical rod, the 

 inner end of which lies in the peri-nephrostomial 

 sinus on the dorsal surface of the testis. This 

 . end is slightly enlarged, and has a cauliflower- 

 like appearance, the cells covering it being spongy 

 and ciliated, and often depressed at their ends. 



This inner end of the testis-lobe probably 

 corresponds to the funnel-like opening of the 

 nephridium into the body-cavity in the earth- 

 worm (p. 68). 



b. The main lobe is the thickest part of the rod : it is 



continuous with the outer end of the testis-lobe, 

 and forms the anterior limb, the bend, and half 

 of the posterior limb of the loop. 



c. The apical lobe is separated by a slight constriction 



from the posterior limb of the main lobe. It runs 

 downwards and inwards, diminishing slightly in 

 thickness, and ends in a coiled-up extremity, the 

 apex of which is closely attached to the junction 

 of the testis-lobe and the main lobe. 



d. The vesick-duct is a short narrow tube arising 



from the anterior limb of the main lobe at its 

 junction with the testis-lobe, and running back- 

 wards beneath the apical lobe to the vesicle. 



e. The vesicle is a somewhat flattened sac with 



muscular walls : it lies behind the rest of the 

 nephridium, and opens to the exterior by a short 

 duct, the opening being on the ventral surface of 

 the body and in the hindmost annulus of the 

 somite in which the nephridium lies. 

 2. Microscopic structure of a nephridium (fig. 22). 

 Eemove a nephridium from a freshly killed specimen; 



