84 FIRST BOOK OF ZOOLOGY. 



by which they are enabled to cling. These are called prop- 

 legs, or false legs. 



It would be well for the pupils to collect some leeches and 

 earthworms, and, if they live near the sea-shore, a few worms 

 may be collected under stones at low tide. 



Having collected these, let the pupils compare them with 

 the larvae of insects. 



As the larva of an insect comes from the egg, it has its 

 full number of segments at the outset. In the larva of a 

 butterfly, for example, there may be counted, besides the head, 

 twelve segments or rings, and this number does not increase 

 as the creature grows, but remains constant ; and, as we have 

 already learned, the creature does not long remain in its 

 worm-like stage, but assumes other conditions, ultimately be- 

 coming a creature unlike, in form and habits, the larval con- 

 dition in which it spent a portion of its life. 



80. The true worm, on the contrary, comes from the egg 

 with a very limited number of segments, and as it grows 

 new segments are formed, till in some worms as many as 

 four or five hundred segments are developed before the ani- 

 mal has attained mature proportions, and in this condition 

 it remains ; that is, it is complete, never changing or passing 

 through larval or pupal stages to develop into something 

 quite unlike the worm. 



Briefly, then, a larva may be distinguished from a true 

 worm generally by its limited number of segments, and, when 

 supplied with legs, having three pairs of jointed ones on 

 the anterior rings of the body. 



