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. 68, 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. si] 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. st) ; on its ventral surface is a trans- 

 verse triangular aperture, the mouth (Mth). The rest of 

 the body is more or less distinctly marked by annular 

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



