76 INTRODUCTION TO ZOOLOGY CHAP. 



cases in which the needle-like leaves of the Scotch pine were 

 used for this purpose ; each pair of leaves was drawn in by its 

 fused base and then the sharp tips of the separate leaves 

 were pressed back into the earth, so that they should not 

 hinder the free movement of the body. Worms often lie for 

 many hours just inside the mouth of their open burrows, and 

 they are apt to do this especially in the early morning hours, 

 probably in order to enjoy the warmth of the sun; this 

 habit leads to their detection and large destruction by birds. 

 " It is the early bird that gets the worm." 



Each burrow terminates usually in a little enlargement 

 which is lined with tiny pebbles or hard seeds, and into this 

 chamber the worm retires for the winter months or during 

 any prolonged drought in the summer. Sometimes several 

 worms remain coiled together during the winter. 



Res iration ^ e neec * * a mo ^ st environment is a very real 

 a ' one to worms. It is absolutely essential that their 

 skins should be kept damp, since there are no special re- 

 spiratory organs, and the aeration of the blood takes place 

 directly through the skin. There are two chief longitudinal 

 blood-vessels, both of which can be seen through the trans- 

 parent skin ; the dorsal vessel runs the whole length of the 

 body above the alimentary canal, and the ventral vessel 

 below it. In each segment, two blood-vessels pass off 

 from the ventral vessel, carrying impure blood to the skin, 

 where it is oxygenated. The purified blood then flows 

 through other vessels to the various organs of the body, 

 and finally is carried to the dorsal vessel, in which it passes 

 forward and is pumped by five pairs of short contractile 

 vessels, lying on either side of the alimentary canal in 

 segments 7 to 11, into the ventral vessel again, whence it is 

 once more carried to the skin. The five pairs of contractile 

 vessels are known as the five pairs of hearts. 



The blood is red, containing haemoglobin, the substance 

 present in the red blood-corpuscles of vertebrate animals ; but 

 in the worm it is not in special corpuscles, but is dissolved in 

 the general fluid or serum of the blood, there being only 

 white corpuscles floating in the red fluid. 



It is the haemoglobin which holds the oxygen and carries 

 it to the different parts of the body, and such an " oxygen- 

 carrier" is specially essential to animals which may suffer 



