CH^ETOPODA 



447 



R. 



width. (How far is this an advantage to the animal ? Compare with 

 other burrowing creatures.) These runs as a rule only extend to a 

 depth of about 20 inches, but in exceptional cases to as much as 6 feet. 

 Owing also to its "vermiform " shape, the animal meets with compara- 

 tively litye resistance in the dense medium in which it lives. (Compare 

 with fishes, p. 266.) 



D. Colour. 



As in most animals living underground (examples), the skin in many 

 worm species (not all) is colourless. The fleshy colour of the worm is due 

 to the blood showing 

 through the body 

 wall. 



E. Motion and Loco- 

 motor Organs. 



(a) The earth- 

 worm progresses by 

 stretching and ad- 

 vancing the fore-part 

 of its body, and then 

 drawing the posterior 

 portion of the body up 

 to it. How are these 

 movements accom- 

 plished ? They are 

 unlike those of snakes 

 or of any other ani- 

 mals. They are ex- 

 plained, however, by 



TRANSVERSE SECTION THROUGH BODY OF EARTH-WORM 

 (SOMEWHAT DIAGRAMMATIC). 



The section is taken in such a position as to show the transverse 

 septum, S., between two adjacent body segments^ R. and 

 L. , annular and longitudinal muscular layers ; B. , setse ; 

 M., muscles which move the setse; D., intestine; Rf. and 

 Bf., dorsal and ventral bloodvessels, from each of which two 

 branches are given off ; N". , the two principal nerve chords, 

 each giving off two nerve branches. 



the peculiar structure 



of the skin. If an earth-worm which has been killed in weak spirit be 

 opened with fine scissors, we shall be struck by the remarkable thick- 

 ness of its " skin." On a closer examination, however, we shall see that 

 we are here not merely dealing with the skin, but with a strong muscular 

 mass intimately fused with the skin, the so-called musculo-cutaneous tube. 

 The muscular portion of this tube is itself composed of two layers : an 

 outer layer, which surrounds the body like a ring (annular muscle layer), 

 and an inner one, in which the muscular fibres run longitudinally (lon- 

 gitudinal muscle layer). If the first of these layers contracts at any 

 particular spot, the body will there become thinner, or, in other words, 

 be stretched in a longitudinal direction ; if next the longitudinal fibres 



