ANiNKLIDA. 



247 



is brought forwards to the head : after which the 

 same series of actions is repeated, beginning with 

 the advance of the head. 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 animals 

 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, or even 

 tentacula, have been given to this simplest of ver- 

 miform animals-t 



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 alone, the Serpula 

 (Fig. 133), forms for itself an external shell, which 



* See Home ; Lectures on Comparative Anatomy, vol. i. p. 114. 



t Jacobson suggests that the Gordius may be an ag-gregation of 

 numerous individual animals enclosed in a longitudinal tubular 

 envelope. (Ann. Sc. Nat. serie 2, i. 320.) 



