72 



INTRODUCTION TO ZOOLOGY 



CHAP. 



in very varied situations, and do not seem to mind whether 

 the soil they live in is rich or poor so long as it is moist. 



Moisture is an essential 

 condition for their life, 

 exposure to drought kill- 

 ing them most rapidly. 

 They are therefore rare in 

 dry sandy soil, and most 

 common in a damp soil 

 covered with short vegeta- 

 tion which protects the 

 surface from the heat of 

 the sun. 



External . The body is 

 Appearance long and nar- 

 and row, shaped 

 Structure. Hke & cylin _ 



der, but tapering at each 

 end, especially in front. 

 It is covered with a soft, 

 naked, slimy skin, which 

 is slightly iridescent, 

 owing to the fact that 

 it is very finely striated. 

 The red colour of a worm 

 is due to the flesh below 

 being reddened by its 

 blood supply and showing 

 through the transparent 

 skin ; the colour is darker dorsally than ventrally ; the chief 

 blood-vessel of the body can be clearly seen as a thin red 

 line running from head to tail along the back ; a ventral 

 vessel is also distinguishable running below the alimentary 

 canal. 



The body is marked with a great number (120 to 180) 

 of circular grooves. These external rings correspond to an 

 internal segmentation of the body into chambers by delicate 

 transverse plates of skin, most of the organs in one such 

 chamber being duplicated in the next, so that the segmenta- 

 tion in these worms is not merely superficial, but extends 

 to many of the vital organs of the body. Below the first 



FIG. 37. Lumbricus terrestris. 



A, View from above. B, View from the side. 

 C, Egg cocoon. D, Two segments seen from 

 below to show the bristles or chaetae ; m, 

 mouth ; cl, clitellum or " saddle " ; a, anus. 



