448 TEXT-BOOK OF ZOOLOGY 



contract, the effect will be to again shorten the body, and in this manner 

 the movements of the body above described are accomplished. 



(fr) The great extensibility and pliancy of this locomotor apparatus is 

 still further increased by its subdivision into a large number of rings, or 

 annular segments (see p. 308) — Annelida, i.e., ringed worms. Corre- 

 sponding to this external annulation we also find the internal body cavity 

 subdivided by delicate transverse septa into a series of chamber-like 

 segments, corresponding to the body rings. 



(c) Motion is further facilitated by the action of minute bristles, or 

 seta, eight of which are found upon each body segment, and which are 

 arranged over the body in four longitudinal rows (two lateral rows and 

 two on the ventral surface). If one of these worms be carefully drawn 

 through the tips of the fingers from behind forwards, the setae may be 

 distinctly felt, while their rustle may be clearly heard if the animal is 

 allowed to creep over some paper. They are inserted in the skin, and 

 are protruded by the action of special muscles, again retracted, and in 

 locomotion are moved from behind forwards like legs. Owing to their 

 shortness, they are unable to raise the body (especially as this is devoid 

 of a skeleton) from the surface of support ; in fact, the body is really 

 dragged along the ground, so that progress is necessarily slow. (Compare 

 p. 308.) Hence the track of the earth-worm may be seen on soft, damp 

 soil as a distinct furrow. These setae also act as " climbing-irons," by 

 means of which the worm holds fast to the walls of its tunnel and climbs 

 up and down in it. Since the soft body of the animal is in the earth 

 supported on all sides (somewhat like that of an aquatic animal), these 

 movements are accomplished with the greatest ease. 



(d) The manner in which the earth-worm, bores its runs is easily 

 observed by placing one of these animals on loose soil. It will then be 

 seen to extend the anterior (cephalic end) of its body, so that it becomes 

 quite thin and pointed ; next, by means of the very sensitive first body 

 segment, or cephalic lobe, it feels about for a crack or scratch in the 

 ground, into which it pushes the anterior end of its body, which at the 

 same time swells considerably and forces apart the soil in the manner 

 of a wedge. The swelling is caused by the muscular pharynx, into 

 which the mouth opens on the ventral surface of the second body 

 segment. In hard soil which it cannot penetrate by burrowing, the 

 earth-worm digs its runs by gnawing. It takes up the soil with its 

 mouth, passes it through the intestine, and returns it by the terminally 

 placed anus to the surface in the form of the well-known worm- 

 castings. 



