4 OLIGOCHAETA 



is an organ of a rather different nature, possibly comparable to the introvert of the 

 Gephyreans. 



In a few earthworms the prostomium seems to be completely absent. Perriee and 

 I myself did not find any trace of the prostomium in Pontoscolex, but Hoest has 

 subsequently affirmed its existence. There appears however to be no prostomium in 

 Deodrilus. Is the absence of the prostomium to be regarded as an archaic character or 

 as due to degeneration 1 The two alternative views Jiave been put forward. They 

 depend of course upon the moi-phological nature of the prostomium, concerning which 

 again there are two current views. 



Wilson in his lately published account of the development of the earthworm holds 

 that the prostomium is a segment; the only difference which it shows froin other 

 segments is that its cavity is unpaired. Wilson explains this by looking upon it 

 as the terminal segment of a body which represents an elongated ring, — the view ih 

 fact that the Annelid's body can be derived from a Coelenterate pulled out lengthwise. 



Vejdovsky (9), on the other hand, found that the prostomium was a comparatively 

 late development — undoubtedly an outgrowth of the first segment. The mouth is 

 originally completely terminal. 



§ 3. External Apertures. In many earthworms various pores upon certain of the 

 segments are visible ; these are (i) the dorsal pores opening into the body cavity, (a) the 

 external pores of the nephridia, (3) and lastly the apertures of the reproductive ducts. 

 The varying position of these pores will be found mentioned under the various organs 

 of which they are the outlets, either in the general part of this work or in the systematic 

 section. A few general observations however may find a place here. The older 

 naturalists used the varying position of these different pores for the purpose of specific 

 and generic definition — usually with questionably useful results ; nevertheless it is 

 possible for a person conversant with internal structure to make a pretty accurate 

 guess by the external characters alone as to the genus, or even in a few cases the 

 species, which he is examining. Most conspicuous of all the external orifices are as 

 a rule the male pores ; it is only in the Geoscolicidae and in some species of Lum- 

 bricus that they are not obvious, for the reason that the sperm ducts do not terminate in 

 a spermiducal gland ; in Allolobophora they are conspicuous, although they do not 

 terminate in such a gland, for the integument is swollen and glandular at their point 

 of opening : in this case the position is always the fifteenth segment ; if the pores are 

 obvious and upon the eighteenth segment the genus will be certain to be one of the Crypto- 

 drilidae or Perichaetidae (which can be further differentiated of course by the setae). 

 If the male pores are double and upon segments seventeen and nineteen the worm 

 will be an Acanthodrilid. In all Eudrilidae the male pores are exceedingly conspicuous ; 



