STRUCTURE OF EARTHWORM OF GENUS DIACH^TA. 163 



varying position of important organs, and apparently confirm- 

 ing it, is the observed fact that the growth of the worm — the 

 addition of new segments — takes places at the tail end. It is 

 common to find worms in which the last segment, or the last 

 two or three, have no setse at all, or fewer than the normal 

 number, while the internal organs show a corresponding em- 

 bryonic condition. In spite, however, of these facts, which 

 are undoubtedly true so far as they go, another mode of for- 

 mation of new segments occurs in earthworms. Fritz 

 MilUer (6), in describing the anatomy of Urochseta core- 

 thrura (termed by him Lumbricus corethrurus), called 

 attention to the invariable presence, at a constant distance from 

 the clitellum, of a group of segments evidently newly formed, 

 for the reason that they were unprovided with setse. Here, 

 therefore, is an example of an intercalary growth of seg- 

 ments ; but as it takes place in the least difi'erentiated part of 

 the body, it is perhaps to be regarded as being essentially 

 difi'erent from the process of formation of new segments at the 

 caudal end. 



The structure of Diachseta appears to me to be 

 suggestive in the light of the hypothesis of an in- 

 tercalary growth of segments at the anterior end. 



It has been mentioned that the first three segments are very 

 much narrower than those which follow, and they can be 

 apparently retracted into the buccal cavity ; it may be, there- 

 fore, that they are in course of disappearance — the initial steps, 

 i.e. the disappearance of the setse and the reduction in size, 

 having already occurred. The opposite interpretation is of 

 course possible. But whatever may be the value of these facts, 

 the arrangement of the setae on the vith, viith and viiith seg- 

 ments (or vth, vith and viith) seems to be inexplicable except on 

 the hypothesis of the intercalary production of new segments. 



Throughout the body the setse, although they are so far 

 irregular in their arrangement that they do not correspond in 

 position in successive segments, are nevertheless regular, in that 



corresponding to the two testes on each side may be the original condition in 

 the Oligochseta. 



