272 University of California Publications. [Zoology 



could be counted at a time. A rectangular diaphram of such 

 dimensions that its width was one-fourth the width of the meta- 

 mere and its length one-eighth the length of the metamere, was 

 put in the eyepiece. By means of a mechanical stage, the whole 

 area of each metamere was gone over, and plots were made of 

 all sense organs in each of these small divisions. The entire 

 surface of one worm was gone over in this way, and parts of 

 four others were enumerated for comparison. 



The work, for the most part, was carried on in the zoological 

 laboratory of the University of California, but was completed 

 in the laboratory of the University of Oregon. The problem 

 was suggested by, and the work carried on under the guidance of. 

 Professor C. A. Kofoid of the University of California, and I 

 wish here to express my sincerest appreciation for his kind 

 advice and for bis help in the final arrangement of the paper. 



STRUCTURE OF THE SENSE ORGANS. 



The structure of the sense organs of Lumbricus and Micro- 

 scolex is practically the same. In Microscolex each organ (Plate 

 XXIV, Fig. 4) is made up of a group of sense cells lying loosely 

 in a cavity surrounded by a layer of boundary cells which form 

 a continuous layer usually one cell thick, but it may be thicker. 

 They are the same as the supporting cells of the epidermis, with 

 the exception that they are usually greatly flattened. The 

 boundary cells are somewhat longer than the epidermal cells, 

 because they are bowed outward and stretch from the elevated 

 area just under the cuticle to the thinned part of the basement 

 membrane beneath. The sense organs are ovoid, the greatest 

 transverse diameter being a little closer to the proximal than 

 to the distal end. The smaller distal end forms a raised spot 

 on the cuticle, which is here thinned and has canals for the 

 passage of the sense hairs. The basal end is usually somewhat 

 flattened, and rests on the basement membrane of the epidermis. 



The size of the organs is about the same for both worms. 

 They measure from 80 to 100 microns in height, 18 to 20 in 

 diameter at the top and 40 to 60 in the widest part. In Micros- 

 colex there is a wider range of variation in the diameter of the 

 top as indicated by the dimensions of the sense areas in the 



