276 University of California Publications. [Zooloc.y 



The third zone is the caudal, or the posterior. It occupies 

 an area between the median and the septal furrow, and covers 

 an arc the convexity of whose surface points posteriorly. The 

 distribution here is exactly the reverse of that in the anterior 

 zone. Excepting in the prostomium and the first metamere the 

 posterior zone has the fewest and smallest sense organs. In 

 this zone the organs decrease in number caudad till we reach 

 the sixtieth metamere, where they begin to increase in both 

 size and number, though they continue to decrease caudad in 

 the other zones, resulting in the approximation to the constant 

 number previously noted. The anterior and the posterior mar- 

 gins of this zone have the organs scattered, the majority being 

 in the central part. 



In all the segments except the first five and the last five the 

 average diameter of the sense organs in the anterior zone is 16 

 microns, in the median they are 19 to 22 microns, and but 10 in 

 the posterior. Again turning to Lumbricus, we find that, begin- 

 ning in the second, third and the fourth metameres, a median 

 line of sense organs is prominent, and diminishes caudad. No 

 distinct median zone is here recognized, the median line of organs 

 in line with the chaetae are not separated from the posterior 

 zone as they are in the anterior, so that only two zones are distin- 

 guishable, a cephalic and a caudal. Around each opening of 

 the dorsal pores, nephropores and sexual ducts of Lumbricus 

 groups of organs were found guarding these entrances. No 

 such distribution was found in Microscolex, each opening, on the 

 contrary, being surrounded by a small clear area containing no 

 sense organs at all (Plate XXV). In both the worms the sur- 

 face of the prostomium and the first metamere are covered with 

 sense organs, so that no distinct zones can be made out. In 

 Microscolex the posterior margin of the first metamere contains 

 many smaller sense organs, while the rest of the surface of the 

 prostomium is covered by very large organs. The following 

 table shows the distribution in the antero-posterior direction in 

 . the metameres of Microscolex. 



