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THE BOOK OF NATURE STUDY 



structural causes similar to those which cause the same appear- 

 ance in soap-bubbles, " starred " ice or glass, and other colourless 

 objects. In this instance the cause of the iridescence lies in the 

 extremely thin, horny covering or " cuticle/' which is outside 

 the true skin and is perfectly transparent, but engraved with two 

 sets of microscopic grooves crossing each other at right angles. 



Movements. When crawling along a worm does not wriggle 

 from side to side as do snakes, but progresses in a straight line. 

 This is accomplished by alternately length- 

 ening and shortening the whole or successive 

 regions of the body. But these rhythmic 

 contractions would achieve no good result 

 nor do they on a perfectly smooth surface, 

 such as wet glass unless there were some 

 means of obtaining a hold upon the soil. A 

 firm grip is, however, secured by the use of 

 small bristles which can be darted out from 

 the lower surface. Each segment has eight 

 of these bristles, arranged in pairs, two pairs 

 on the right and two pairs on the left side. 

 They can be plainly felt if a worm is pulled 

 between the finger and thumb, and, being 

 about T V inch long, they can be seen by the 

 naked eye. This is best managed as 

 follows : hold the worm in the hand on a 

 level with the eye, and allow it to crawl 

 forward between the finger and thumb so 

 that it reaches out into the air ; as soon 

 as the worm has got the front region of the body extended 

 as far as it can it will dart out the bristles in the endeavour 

 to get a hold upon something and to pull the hinder region 

 forward. That is exactly what the worm does when naturally 

 crawling on or through the earth ; the front region is pushed 

 forward, its bristles are then darted out and catch on in- 

 equalities of the surface like so many pegs, the hinder region is 

 then pulled forward, pegged firm, and so on. In the region of 

 the " saddle " and certain other segments between it and the 

 head the bristles are longer, and differ in shape from those else- 



B 



FIG. 61. B, ordinary 

 seta ; C, specialised 

 (genital) seta of earth- 

 worm. 



