WORMS. 159 



species build the tube in a coil, looking very much like a 

 snail-shell, as in Fig. 147, B. This form is very common on the 

 ordinary rock-weed, and may be collected in the debris thrown 

 up by the waves. Pupils should collect these, and, if possi- 

 ble, watch the little creatures as they protrude the feathery 

 appendages which surround the head. In the forms figured, 

 one of the appendages is modified into a sort of plug, and, 

 when the worm retires within the tube, the plug stops up the 

 aperture securely, as certain snails close the aperture of their 

 shells with an operculum. 



144. In studying the affinities existing between animals 

 in order to group them together naturally, the pupils should 

 learn how unsafe it is to judge by the external appearances of 

 the animals to be classified. For example, the little worm B 

 just described has a coiled, limy shell, which might easily be 

 mistaken for the shell of a snail. Yet the slightest examination 

 of the soft parts within shows that the animal is made up of 

 segments, and that, minute as it is, there are bunches of 

 bristles, or setae, projecting from the sides of the segments, 

 and from these and other characteristics the creature is proved 

 to be a true worm, having no affinities with the snails. The 

 barnacles too have a limy shell; yet, when the creatures 

 within are examined, their affinities with the crustaceans are 

 seen at once ; and, although distinguished naturalists in past 

 times grouped them with the shell-fish, or moll asks, they 

 properly belong to that class which includes the lobster and 

 crab. 



