NEREIS AND ITS ALLIES 



151 



uniform crimson color and a large number of cirri massed 

 at the head. The tubes made by some of the Terebellidee 

 are very beautiful. The tube of Cistenides (Fig. 138) is 

 found in the sand under stones and is composed of grains 

 of sand cemented together and regularly arranged so as to 

 form a firm wall. 



Finally, Serpula 1 secretes crooked, round calcareous 

 tubes, which may be found adhering to stones near low 

 water (Fig. 140). 

 From the mouth of 

 the tube the head, 

 with its tentacles, 

 may be protruded, 

 but it quickly re- 

 tracts from danger 

 and closes the open- 

 ing of the tube as a 

 marine snail does its 

 shell, by means of an 

 operculum or lid. 



Some worms have 

 gained a parasitic 

 habit, and in con- 

 sequence have become much modified in form and struc- 

 ture. Such is the case with some of the round- 

 worms. Some of these are thread-like, live in springs 

 or pools, and are regarded by the uninitiated as animated 

 horse-hairs. Others are spindle-shaped, as for example 

 the "vinegar eel" and the round- worms that are common 

 in stagnant water. Others live in the food canal, as for 

 example the stomach worm (Ascaris) of the horse and the 



1 Diminutive of serpens, serpent. 



FIG. 140. Serpula tube on a bit of oyster 

 shell that is perforated by the boring sponge. 

 The tube lies in the centre of the figure. 

 Nat. size. Photo, by W. H. C. P. 



