278 University of California Publications. [Zoology 



from 6 : 5 to 5 : 3. The greatest contrast is in the middle of the 

 worm, where the ventral organs out-number the dorsal 5 to 3. 



The distribution of the sense organs in Microscolex is signifi- 

 cant when viewed in the light of the habits of this worm. Micros- 

 colex is a small, frail worm, and its movements are quiet and 

 rapid. Backward movement is almost as free as movement in 

 the other direction. Many times during experiments on these 

 worms they have covered a distance of more than half a meter 

 in a continuous movement. While the common Ailolobophora 

 calignosa has the ability to move backwards, it never shows this 

 in so marked a degree as does Microscolex elegans. I have seen 

 these worms moving backwards freely, not only in the burrows, 

 but also on free surfaces, when no experiments were being tried 

 and there was apparently no occasion for such action. Natu- 

 rally, we should expect the anterior end of the worm to be the 

 most sensitive, and therefore have more sense organs than do 

 the other parts of the worm. There being in Microscolex this 

 backward movement, the posterior end should also be particu- 

 larly sensitive. Actual count of the organs of these regions 

 shows an increase in the number of organs posteriorly and on 

 the posterior arc of the rear segments. The anterior segments 

 have the largest numbers, which decrease till a region of con- 

 stancy occurs behind the clitellum; then follows an increase in 

 the numbers from the nintieth segment on to the end of the 

 worm. 



Another noticeable habit of this worm is its marked squirm- 

 ing movement, especially noticeable on a smooth surface. There 

 are two reasons why the lateral zones should have more sense 

 organs than the dorsal or the ventral zones. First, along the 

 sides of the worm the nephropores occur. If these pores are 

 places of great sensitiveness, then we should expect to find, as 

 in Lionbricus, each nephropore provided with a distinct group- 

 ing of the sense organs. As Plate XXV, Fig. 6, plainly shows, 

 there is no grouping at all about these openings. On the con- 

 trary, the area just about the pore is usually quite free from 

 organs. There are no other openings on the lateral surfaces of 

 the body. The lateral sense organs are scattered in Microscolr.r, 

 and the whole surface must do the duty that a group of the 



