LESSON XXV 



POLYGORDIUS 



POLYGORDIUS is a minute worm, about 3 or 4 cm. in length, 

 found in the European seas, where it lives in sand at a 

 depth of a few fathoms. It has much the appearance of a 

 tangle of pink thread with one end produced into two delicate 

 processes (Fig. 67, A). These, which are the tentacles, mark 

 the anterior end of the animal the opposite extremity* 

 which in some species also bears a pair of slender processes, 

 is the posterior end. As the creature creeps along, one side 

 is kept constantly upwards and is distinguished as the dorsal 

 aspect ; the lower surface is called -ventral. 



The anterior end is narrower than the rest of the body, 

 and is marked off behind by a groove (B and c) ; this 

 division is called the prostomium (Pr. st) and bears the 

 tentacles (/) already mentioned in front and above ; and on 

 each side a small oval depression (c. p) lined with cilia. 

 Immediately following the prostomium is a region clearly 

 marked off in front, but ill-defined posteriorly, and known as 

 the peristomium (Per. sf) ; on its ventral surface is a trans- 

 verse triangular aperture the mouth (Mtti). The rest of 

 the body is more or less distinctly marked by annular 

 grooves (D and E, gr] into body-segments or metameres 



