EXCEETOEY SYSTEM 47 



j)lace it on a slide in a drop of 0"75 per cent, salt solution, 

 and spread it out with needles so as to show the mutiMl rela- 

 tions of its several parts. 



Bemove another nephridium in the same way, and tease it 

 slightly. 



Cover the specimens and examine with low and high 

 powers. 



a. The testis-lobe consists of a spongy mass of cells, 



traversed in an irregular manner by ductules. 



b. The main lobe consists of large granular cells, 



traversed in a very complicated manner by fine 

 branching ductules which pass across from cell 

 to cell, and are lined by a cuticular layer. 

 0. The apical lobe consists of cells traversed by much 

 larger ductules, most of the cells being merely 

 hollow cylinders placed end to end to form tubes. 



d. The main duct of the gland is also formed of cylin- 



drical cells placed end to end, the passage being 

 intracellular. It commences at the apex of the 

 apical lobe, passes back along this lobe and along 

 the posterior and anterior limbs of the main 

 lobe, enters the apical lobe again, and goes right 

 round the loop a second time, and then passes 

 as the vesicle-duct to the vesicle. 



e. The vesicle, which is extremely dilatable, possesses 



a muscular coat, and is lined by a ciliated epithe- 

 lium, as is also the short passage leading from it 

 to the exterior. 



f. The blood-vessels form a very fine capillai'y plexus 



lying between the individual cells of the nephri- 

 dium. 



3. Special characters of the nephridia. 



a. The nephridia in the second to the seventh somites, 

 in which there are no testes, have the testis-lobe 

 short or absent, and devoid of the cauliflower- 

 like end. These anterior nephridia are smaller 

 than the succeeding ones. 



