VERMES 



163 



tract and the longitudinal muscles relax, the worm be- 

 comes long and thin. It holds fast by means of stiff 

 hairs called setw (Fig. 

 92) which are planted in 

 such a direction that the 

 animal can not move 

 against them. 



Surface Characteris- 



Fig. 92. — Diagram to illustrate the 

 action of the setae, m, muscles ; «, 

 seta; w, body wall. Dotted lines rep- 

 resent the position of the seta and 

 its muscles when bent in the opposite 

 direction. (From Sedgwick and Wil- 

 son.) 



tics. — These setae may 

 easily be seen with the un- 

 aided eye stretching in 

 two double rows the length 

 of the ventral surface 

 (Fig. 93). Also on this surface are little swellings 

 that mark tiny openings. These are situated between 

 the ninth and tenth, between the tenth and eleventh, on 

 the fourteenth and on the fifteenth rings. The mouth 

 is under an overhanging lip at the anterior end, and 

 directly opposite in the posterior end is the anal open- 

 ing. A swollen girdle encircles the middle of the body. 



One line only can be drawn on the surface that will 

 divide the worm into two similar parts. It is there- 

 fore bilaterally symmetrical. The repetition of similar 

 rings throughout its length gives it another kind of 

 symmetry called serial symmetry. As in the starfish this 

 external repetition of similar parts indicates a corre- 

 sponding internal repetition. 



Assimilation. — A slit down the dorsal side of the ani- 

 mal and a few pins to hold the flaps of skin open will lay 

 bare the interior of the animal. The alimentary canal, 

 which is plainly a tube open at both ends, extends 



