348 OLIGOCHAETA 



Some of the aquatic species afforded material to Bonnet and 

 Spallanzani for their experiments upon the powers of regenera- 

 tion of the animals when cut into fragments, while the work 

 of O. F. Mliller 1 upon various Naids is a monument of careful 

 anatomical description. Our knowledge of the aquatic Oligo- 

 chaeta does not appear to have advanced so rapidly as has that 

 of the earthworms. 



External Characters. The most salient external characteristic 

 of this group of worms, which vary from 1 mm. to 2 metres in 

 length, is of course the segmentation. The entire body is 

 divided into a number of rings, which are for the most part 

 similar to each other ; a fragment of an earthworm's body could 

 not be accurately replaced unless it had been cut from the 

 anterior region. There is precisely the same regular segmenta- 

 tion in the aquatic representatives of the Order. At the anterior 

 end of the body in the common earthworm (and in nearly all 

 Oligochaeta) is a small unpaired lobe, which overhangs the 

 mouth, and is usually termed the prostomium ; the mouth itself 

 is surrounded by the first segment of the body, which never bears 

 any chaetae in any Oligochaete. The prostomium is occasionally 

 greatly developed, and in such cases doubtless forms a tactile 

 organ of importance. This is especially the case with the South 

 American genus Rhinodrilus, where the lengthy prostomium can 

 be retracted at will. The aquatic JVais lacustris ( = Stylaria pro- 

 loscidea) has also an exceedingly long prostomium, which cannot, 

 however, be retracted, though it is contractile. At a certain dis- 

 tance from the anterior end of the body, fixed for the species, but 

 varying greatly from genus to genus and from species to species, 

 is the clitellum. This region of the body (popularly believed to 

 mark the spot where a worm divided by the gardener's spade has 

 come together again) is associated with the reproductive function, 

 and serves to secrete the cocoon in which the creature's eggs are 

 deposited. It has in the earthworm a thick glandular appear- 

 ance. A more minute examination of the worm's body will show 

 the orifices of the reproductive ducts and of the excretory organs 

 which will be found described below. In addition to these, 

 all British earthworms and a large percentage of the tropical 

 forms have a row of pores along the back, which are between the 

 successive segments in the median line. These " dorsal pores " 



1 Naturg. ein. Wurm-Arten d. siissen u. salzigen Wasser, Copenhagen, 1771. 



