ZooL.— Vol. II.] RISEN— OLIGOCH.'^TA. lOI 



But what would be the advantage of such a zone? Undoubt- 

 edly a greater sensitiveness at the point of the tail of the 

 worm where it projects out of the mud or soil, Pontoscolex 

 is mainly a mud worm, one ^}^^ca^%, Pontoscolex Lilljborgii^ 

 being Limicolide in its habits. The species under discussion 

 was found to be common in wet places, — along river banks, 

 etc., where the mud was barely covered by water. 

 The least movement of the ground causes the worms to 

 retire rapidl}^ down the burrows. Like all other aquatic 

 Oligoch^ta, the favorite position of Pontoscolex is with the 

 tip of the tail extending above the mud, making the caudal 

 zone its point of contact with the surface of the soil. 

 It is evident that were this point especially sensitive it 

 would be of great advantage to the worm, enabling it to 

 quickly detect vibrations caused by the approach of birds or 

 other enemies, and giving it time to retreat into the burrow. 



But I believe the caudal zone to possess also another 

 function of no small importance. It is especially rich in 

 muscular fibres (fig. 21), much more so than the somites 

 adjoining the zone. Upon several occasions I have observed 

 the caudal zone to bulge out and increase in width when 

 the live worm was touched. This swelling up of a portion 

 of its body would undoubtedly help the worm to retain its 

 hold in the ground, and prevent its being easily pulled out. 



Protective Sti'uctitres. — Other devices which enable this 

 worm to retain its hold in the ground are the peculiar vacu- 

 oles in the anterior somites and the corkscrew twist of the 

 body. The former enables the worm to suddenly extend 

 the muscular layers and to hug the soil closer. The cork- 

 screw twist must serve the same purpose. 



The extensive distribution of Pontoscolex would indicate 

 that it must be unusually well protected. To the protective 

 organs just described we must also add the intratyphlosolar 

 canals which enable the typhlosole to discharge into the 

 intestine; these canals will be described later. To reca- 

 pitulate, the organs and structures which are especially 

 favorable to the survival and consequent distribution of 

 Pontoscolex are as follows: — 



