198 



THE MECHANICAL FUNCTIONS. 



ceeding segments are necessarily elongated: these are next 

 contracted; and so on, in succession, till the whole 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 back- 

 wards, or with the tail foremost.* 



Great variety exists in the forms of the animals referrible 

 to the type of Annelida. The Gordius, or hair-worm, (Fig. 

 ] 32,) is that which exhibits the greatest development 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 animals. 



135 



Many of the animals of this class being soft and defence- 

 less, are obliged to consult their safety by retreating into 

 holes and recesses, or by burrowing in the sand or mud. 

 One genus only, the Serpula (Fig. 133,) forms for itself an 

 external shell, which is shaped into a spiral tube. Others, 

 as the Sabella and the Terebella, accomplish the same ob- 

 ject by collecting grains of sand, or fragments of decayed 

 shells, or other substances, which they agglutinate together 

 by means of a viscid exudation, so as to form a firm defen- 

 sive covering, like a coat of mail. Fig. 134 shows this 

 rude architecture in the Terehella conchilega. These co- 

 verings, however, composed as they are of extraneous ma- 



* See Home? Lectures on Coipparalive Anatomy, Vol. i. p. 114, 



