274 University of California Publications. [Zoology 



the very small and numerous openings found all over the cuticle. 

 They are about the same size as the openings for the sense hairs. 

 The cuticle is made up of two layers, an outer dense and an 

 inner loose, fibrous layer. The thinning of the cuticle for the 

 sense organ is at the expense of the inner layer. The outer 

 layer contains two sets of fiber, which cross each other at an 

 angle of about 60 degrees, each fiber running spirally about the 

 worm. The openings for the gland cells and the sense hairs are 

 made by pushing the fibers to one side, while for the larger 

 openings there is a distinct rupture of the fibers. 



DISTRIBUTION OF THE SENSE ORGANS. 



The sense organs were found in all parts of the surface of 

 the body of Microscolex, while in ZiV/mbricus none were found 

 in the clitellum. The organs were not distributed equally, cer- 

 tain regions having more than others. The largest numbers 

 are present in the anterior segments. Passing caudad they 

 gradually decrease till in the middle region of the worm (seg- 

 ments 18 to 90) they reach a. degree of constancy for each suc- 

 cessive segment approximating 220. Continuing caudad beyond 

 segment 90, the numbers again increase, but do not become so 

 abundant as in the anterior segments. Thus in one worm the 

 second segment had 511 sense organs, the thirty-fifth had 218, 

 while the ninety-third had 310. The ninety-third was next to 

 the last segment in this worm. The lowest number in any seg- 

 ment was 136, in the fiftieth ; the highest was 569, in the fifth. 

 In Lumbricus the anterior metameres have the largest numbers, 

 but from here on to the caudal end there is a gradual and unin- 

 terrupted diminution in numbers. The total sense organs in 

 corresponding metameres are unequal. Microscolex elegans con- 

 tains on an average 103 segments, while Lumbricus has over 150. 

 The following table shows some of the figures for corresponding 



segments : 



Prostomium 



and 1st Seg. 10th Seg. 56th Seg. 



Lumbricus agricola 1900 1200 799 



Microscolex elegans 536 324 218 1 



1 Tliis is the number of sense organs in the thirty-fifth segment, which 

 corresponds with the fifty-sixth of Lumbricus, whose length is twice that of 

 Microscolex. 



