Vol. l] Bovard. — Sense Organs in Microscolex Elegans. 277 



Number of Sense Organs in Zones. 



Segment. Anterior. Median. Posterior. 



6 293 135 101 



8 228 83 70 



10 180 81 63 



12 148 73 52 



'14 1D1 69 34 



20 102 64 57 



30 92 58 65 



40 116 57 74 



50 53 32 58 



60 78 72 100 



90 82 76 107 



91 67 61 105 



=93 88 88 134 



The dorso-ventral distribution was not mentioned in Lum- 

 bricus, but is a striking feature of Misroscolex, where four 

 regions — a dorsal, two lateral, and a ventral — may be distin- 

 guished. The dorsal surface always has fewer organs than the 

 ventral, while the lateral surfaces always have more than either 

 the dorsal or the ventral. For example, metameres 10 to 20 

 contain on the dorsal surface a total of 483 sense organs, on 

 the ventral 632, and on one of the lateral zones 724. Recording 

 these as they occur in different metameres of a Microscolex, we 

 have the following table : 



Number of Number of Sense Organs on Surface. 



Segment. Dorsal Lateral. Ventral. 



10 72 76 60 



12 47 80 75 



13 44 60 58 



15 48 63 55 



17 19 63 45 



20 29 71 60 



35 27 67 36 



50 18 50 31 



80 30 61 55 



93 39 89 78 



The only exception to this type of dorso-ventral distribution 

 is in the prostomium and the last segments, and here the organs 

 are more evenly scattered over the surface. The ratio of the 

 number of organs in the ventral zone to that of the dorsal varies 



1 Clitellum. 



■' Next to 'last segment. 



