276 



THE MECHANICAL FUNCTIONS. 



Worms often reverse this motion, and are thus 

 enabled to move backwards, or with the tail 

 foremost. * 



Great variety exists in the forms of the ani- 

 mals referable to the type of Annelida. The 

 Gordius, or hair-worm (Fig. 132) is that which 

 exhibits the greatest developement in length 

 compared with the breadth of the body. It has 

 the form of a very long and slender thread : the 

 annular structure being indicated only by very 

 slight transverse folds of the integuments. No 



external members, nor even tentacula, have 

 been given to this simplest of vermiform ani- 

 mals. 



Many of the animals of this class being soft 

 and defenceless, are obliged to consult their 

 safety by retreating into holes and recesses, or 

 by burrowing in the sand or mud. One genus 



* See Home ; Lectures on Comparative Anatomy, Vol. i. 

 p. 114. 



