STRUCTURE OF THE NEPHRIDIA OF DINOPHILUS. 537 



tlie stomach. Here its solenocytes project into the lumen of 

 the space, as in the case of the former organs. In this 

 nephridium all three parts of the structure reach their 

 fullest development, the solenocytes being most numerous, 

 the second portion being large, and the third portion remark- 

 able for its great length (fig. 10). The thick granular wall 

 of the second portion immediately catches the eye on the first 

 examination of the preparation, its dark red pigment and its 

 hyaline granular walls render it striking. The greater part 

 of its canal lies dorsal to the main mass of the testis. It 

 pierces this to run ventral to it about its outer third, termi- 

 nates near the median line on the ventral surface as in the 

 previous organs, and like them in the segment following that 

 to which it properly belongs. The course of this nephridium 

 is shown in fig. 10. It terminates in the basement membrane, 

 as shown in fig. 7. 



V. The Fourth Nephridium. 



The head of the fourth nephridium is situated opposite the 

 posterior end of the stomach about its junction with the gut. 

 It lies for the most part ventral to the testis, running back- 

 wards and outwards to terminate on the ventral surface near 

 the median line. Its solenocytes project into the posterior 

 prolongation of the corners of the body-cavity space, and are 

 somewhat difficult to see on account of the stomach usually 

 folding over and hiding them when the animal is compressed. 

 It is more feebly developed than the third nephridium, and 

 from the denseness of the testis to which it lies ventral its 

 canal is difficult to follow. It terminates beneath the surface 

 of the epidermis in a manner similar to the other nephridia. 

 The second portion of the nephridium is not marked out by 

 the masses of red pigment as in the previous cases, its walls 

 are but slightly granular. The canal passes among several 

 large clear spaces crowded with spermatozoa. With these it 

 never shows any connection. 



