INVERTEBRATA. 341 



6. The excretory system of the earthworm is of great 

 interest. It consists of a paired metameric series of 

 tubules, which open at one end into the ccelom, and at 

 the other to the exterior. Like the excretory tubules 

 of Amphioxus these tubules are called nephridia. A 

 diagrammatic view of a nephridium is given in fig. 

 15"^Tbe internal end is a ciliated funnel (nephrostome) 

 lying near the mid-ventral line, just in front of a septum, 

 and with its open end facing forwards. This passes back 

 at its narrow end into a tube formed of a single row of cells, 

 the duct perforating the individual cells instead of being 

 surrounded by an epithelium in other words, it is intra- 

 cellular. This tube runs through the septum into the 

 next segment behind, where the rest of this nephridium 

 lies. The tube is looped dorsalwards and ventralwards 

 repeatedly, in a complicated manner, and shows three 

 portions a first where the duct is very fine, a second where 

 it is larger and ciliated, and a third where it is still larger 

 and not ciliated, the protoplasm around it being actively 

 secretory. Finally, there is a portion where the duct is no 

 longer intra-cellular, but lined by an epithelium, and outside 

 this are muscle-fibres, by whose contraction the excretory 

 liquid may be expelled through the external aperture. The 

 secretory part of the nephridium is surrounded by a plexus 

 of capillaries, and here doubtless urea (or some similar 

 compound) is removed from the blood. Through the 

 nephrostome iti is obvious that the coelomic fluid must be 

 continually driven by the cilia towards the exterior, and it 

 may be that some waste-products may be removed in this 

 way, while it is certain that effete cells are thus removed. 

 In this, as in many other cases, however, our knowledge of 

 the details of physiology is far inferior in the case of 

 Invertebrates to what it is in that of Vertebrates. 



7. Nervous System (fig. 177). As has been already 

 mentioned, the main nerve-cord runs in the mid-ventral 

 line, internal to the body-wall. In addition to this there 

 is a pair of ganglia in the head-region on the dorsal side 

 of the alimentary canal. These ganglia, which are small 

 pear-shaped bodies lying side by side in the depression 



