280 University of California Publications. [Zoology 



the comparison of the anterior and the middle pai'ts of the 



worm. The ratio of the sense organs in the two regions is 3 : 1, 



while the ratio of the time reactions is 4 : 9. In the following 



table some of the time reactions are given, the time being given 



in seconds: 



Time Keactions of Different Portions 

 Stimulant. of the Worm. 



Anterior. Middle. Posterior. 



Alcohol 2 7.8 5.6 



Alcohol 3.8 10.2 7 



Alcohol 1.8 10.8 8.6 



Alcohol 4.8 8 8.6 



Alcohol 5 6 4 



Quinine 4.8 5.6 5.6 



Quinine 4.6 7.8 6.2 



Quinine 7.6 8.6 13.6 



Quinine 6 13.6 9 



SUMMARY. 



1. The anterior metameres contain the greatest numbers of, 

 and the largest sense organs. 



2. In the middle of the worm (metameres 18 to 90) a region 

 of constancy is found where each metamere contains about 220 

 sense organs. 



3. Toward the posterior end the number of sense organs per 

 metamere increases to about two-thirds that found in the ante- 

 rior end. 



4. Every metamere shows an antero-posterior and a dorso- 

 ventral distribution of the sense organs. 



(a) In the antero-posterior direction there are three 

 zones — anterior, median and posterior. 



( b ) In the dorso-ventral direction there are four areas — 

 two laterals, a dorsal and a ventral. The number of 

 sense organs in the ventral area exceeds that in the 

 dorsal, and that in the lateral surpasses the ventral. 



5. In the posterior end of the worm the antero-posterior dis- 

 tribution is just reversed, the larger number of organs being in 

 the posterior arc of the metamere. This distribution is eorre- 

 Lated with the habits of the worm. 



6. The largest sense organs are in the prostomium, and the 

 smallest in the elitellnm. 



