THE CCELOMATA INVERTEBRATA 181 



(3) The succeeding part, the wide tube, is still wider than the middle 

 tube, and starts at the distal extremity of the second loop in a 

 distinct vesicular enlargement, the ampulla. This tube also has 

 coloured and very granular walls, for which reason it is sometimes 

 distinguished as the glandular tube. It runs to the base of the 

 second loop and then back through the whole of the first loop to 

 near the point of entry of the narrow tube, where it finally leaves the 

 first loop and passes over into the last part of the nephridium. 



(4) The terminal portion, constituting the entire third loop, is the 

 muscular duct or bladder. It opens to the exterior by the nephridio- 

 pore, and, unlike the other tubes, has a muscular wall forming a sac, 

 in which the excretory fluid can be stored and ejected to the exterior 

 from time to time. The three loops are bound together by con- 

 nective tissue, with which is associated a large number of chloragogen 

 cells, and in which is found a capillary plexus that is particularly 

 marked in the portions occupied by the ciliated and glandular tubes. 



An important difference is to be noted between the muscular 

 duct and the remaining parts of the nephridial tubes. It is lined by 

 flattened cells, so that its cavity is intercellular, whereas the lumen 

 of all the other portions is intracellular. They are actually com- 

 posed of a series of hollow cylindrical cells joined end to end, which, 

 from their appearance, are known as drain-pipe cells. This difference 

 is reflected in their development also, the muscular duct arising as 

 a hollow invagination of the ectoderm, while the remaining portion 

 is laid down as a solid strand of ectodermal cells, in which the lumen 

 is formed later. 



Excretion in these organs is of a twofold nature. The 

 ciliated and wide tubes are granular in appearance, owing to the 

 presence in them of excretory particles that they have formed by 

 secretion from the blood. These are passed into the lumen and 

 washed down into the bladder by the current set up by the cilia, 

 and so in this way nitrogenous waste and water are eliminated. 

 The other kind of excretion is possible, owing to the fact that the 

 nephrostome opens directly into the coelomic fluid, and the cilia 

 covering the marginal cells work in such a way as to convey any 

 small particles to its opening, which is, however, too small to allow 

 of the passage of amcebocytes. Any foreign bodies, bacteria, etc., 

 that may have entered through the dorsal pores can be removed in 

 this way. The main bulk of the matter removed by this channel, 

 however, is derived from the chloragogen cells. These gradually 

 secrete within themselves granules of excrement, and when full come 

 loose from the gut wall into the coelomic fluid. Here they break 

 down into tiny fragments, which are gradually swept down the 

 nephrostome by the action of the cilia. The amcebocytes also 



