xxv GENERAL CHARACTERS 273 



(Mtmr\ the number of which varies considerably. Poly- 

 gordius is thus the first instance we have met with of a trans- 

 versely segmented animal. The last or anal segment 

 (E, An. seg) differs from the others by its swollen form and 

 by bearing a circlet of little prominences or papillae (p) ; it 

 is separated from the preceding segment by a deep groove, 

 and bears at its posterior end a small circular aperture, the 

 anus (An). 



Polygordius may therefore be described as consisting of a 

 number of more or less distinct segments which follow one 

 another in longitudinal series ; three of these, \beprostoinium, 

 which lies altogether in front of the mouth, the peristomium^ 

 which contains the mouth, and the anal segment, which 

 contains the anus, are constant ; while between the peri- 

 stomium and the anal segment are intercalated a variable 

 number of metameres which resemble one another in all 

 essential respects. 



Polygordius feeds in much the same way as an earth- 

 worm : it takes in sand, together with the various nutrient 

 matters contained in it, such as infusoria, diatoms, &c., by 

 the mouth, and after retaining it for a longer or shorter time 

 in the body, expels it by the anus. It is obvious, therefore, 

 that there must be some kind of digestive cavity into which 

 the food passes by the mouth, and from which effete matters 

 are expelled through the anus. Sections (Fig. 68) show 

 that this cavity is not a mere space excavated in the interior 

 of the body, but a definite tube, the enteric canal (A,. B), 

 which passes in a straight line from mouth to anus, and is 

 separated in its whole extent from the walls of the body 

 (A, B. W.} by a wide space, the body cavity or ccelome (ccel). 

 So that the general structure of Polygordius might be imi- 

 tated by taking a wide tube, stopping the ends of it with 

 corks, boring a hole in each cork, and then inserting through 



T 



