THE ANATOMY. NEPHRIDIA 39 



I speak of 'two series' because in the Acanthodrili at any rate the structure of the 

 nephridium differs in relation to the varying position of its external orifice. When 

 the nephridia open in front of the ventral seta they are provided with a very large 

 caecum ; on the other hand, the dorsally opening nephridia have either no caecum 

 at all or the large muscular terminal sac of the nephridium is prolonged a little 

 way beyond the external pore. 



Alternation in position of external pores. In addition to the worms just referred 

 to, a curious condition of the nephridia has been described by Hubrecht in the genera 

 Lumbricus and Allolobophora ^ ; the position of the nephridio-pores shows an alternation 

 similar to that of the worms already described, but there is no change in the position 

 of the nephridia themselves within the body, such as occurs in Acanthodrilus for 

 example. 



The bare fact of the alternation of these pores has been previously referred 

 to by BoEELLi ; Borelli found that in a considerable number of different species 

 this alternation from segment to segment was typical ; the following is a list of 

 these species: — 



Lumbricus rubellus, HofFm. 

 Lumbricus purpureus, Eis. 

 Lumbricus herculeus, (Sav.). 

 Allolobophora turgida, Eis. 

 Allolobophora chlorotica, (Sav.). 

 Allolobophora transpadana, Rosa. 

 Allolobophora complanata, (Duges.). 

 Allolobophora foetida, (Sav.). 

 Allolobophora celtica, Rosa. 

 Allurus tetraedrus, (Sav.). 



To this list others have since been added — for example Rosa's Allolobophora 

 tellinii. In fact it may be taken apparently that the typical arrangement for the 

 genera Lumbricus and Allobophora is the one that has just been referred to. The 

 position of the pores does not show precisely the same kind of irregularity that has 

 been mentioned in Acanthodrilus. Borelli found, and he has been confirmed by 

 Hubrecht, that the pores may either occupy the position that is generally assigned 

 to them in the text-books, i.e. just above the second seta, or they may lie in relation 

 to the fourth seta or finally between the fourth seta and the dorsal pore. Moreover 

 there is no regularity in the alternation from segment to segment and not always 



^ I can confirm from my own observations the accuracy of Hubbecht's statements and figures. 



