i8o 



AN INTRODUCTION TO ZOOLOGY 



cells, covered on their dorsal surface with long fine cilia, they are 

 longer in the middle and reduced as they pass towards the side of the 

 crescent. The actual opening is also crescentic, being limited by 

 the central cell vent rally and a projecting lip dorsally. It leads 

 into a narrow ciliated tube, the neck, which after a short distance 

 perforates the septum and passes on into the next part of the 

 nephridium. This tube is surrounded by a sort of collar composed of 

 a syncytium, that is to say, a mass of protoplasm not divided up into 

 discrete cells, but containing a large number of nuclei. 



The postseptal, or the main portion of the nephridium, is a very 

 long coiled and looped tube, in which we can easily distinguish four 

 distinct regions, (i) The narrow tube is the direct continuation of 



A. 



FIG. 58. A, Diagram of a nephridium of the earthworm. 



ns., nephrostome ; s., septum ; ct., narrow ciliated tube ; m.t., middle tube ; gl., wide grandular 

 non-ciliated tube ; m.d., muscular duct. B, a portion of A enlarged to show the passage from the 

 middle tube to the glandular tube. C, a nephrostome enlarged ; m., marginal cells ; c.s., central 

 cell ; g.c., grooved cells. (After Benham.) From Bourne. 



the preseptal tube, and it runs the complete length of the first and 

 second loops returning to its starting-point. It remains narrow 

 throughout, and is much twisted and coiled upon itself, and is 

 ciliated at any rate in parts of its course. At its end it finally leaves 

 the first loop and passes to the inner point of the second loop, 

 becoming transformed as it does so into the next portion. (2) The 

 ciliated or middle tube is much shorter, starting at the base of the 

 second loop and only running out to its lateral extremity. It is 

 wider than the former, and owes its name to the fact that it is 

 particularly well supplied with long cilia, which remain active and 

 can be seen working vigorously under the microscope for a consider- 

 able time after the nephridium has been removed from the body. 

 The walls of this tube are very granular and of a brownish colour. 



