if HE WORM AND LEECH TRIBE. 313 



bodies. The shell is exuded from 

 the body of the animal just as the 

 covering of a Crustacean is. In 

 the figure one of the animals is 

 stretched up out of its shell, spread- 

 ing forth its delicate gill -tufts 

 which are arranged around its 

 head. It can withdraw itsell en- 

 tirely within the tube, and when 

 it does so there is a provision for 

 shutting it up. You see that one 

 of the long filaments is expanded 

 at the end into a flat, circular disk. 

 This is the door which shuts down 

 on the mouth of the tube after the 



Fig. 246. -Group of Serpula, ^^ filaments ^ ^ drawn m> 



537. There are other animals of this group which, in- 

 stead of having a tubular shell exude from their bodies, 

 form one by connecting together, with a gummy sub- 

 stance from the mouth, particles of shell, sand, small peb- 

 bles, etc. They are in this respect like the larvae of the 

 Caddice-fly, 459. The Terebella, Fig. 247, does this. 



It is here represented 

 with its tentacula ex- 

 =^-~ tending out from the 

 tube. These are used 



Fig.247._Terebellainit S Tube. ^ gathermg i ts food. 



If you take a Terebella, and, breaking up its tube care- 

 fully, get the animal in its naked state, you can, by 

 placing it in some moist sand, see the process by which 

 it forms a new tube. In doing this it takes each grain 

 into its mouth, and then, turning its head backward, 

 places it in its proper position. 



538. The third order is that of the Terricola, so called 

 because they live in the earth. The Earthworm works 

 through the ground by insinuating its pointed head be- 

 tween the grains of dirt, pushing itself forward by some 



