THE TOPOGRAPHY OF ORGANS IN TYPICAL SEG- 

 MENTS OF HIRUDO. 



BENNET M. ALLEN. 



I wish, first of all, to acknowledge my indebtedness to Prof. 

 C. O. Whitman for the incentive to this work, and for much kindly 

 assistance. The work was begun in the hope that it might throw 

 some light upon the question of the limits of the somite in the 

 leeches. No attempt will be made to discuss this question. I 

 shall merely endeavor to describe the position of organs in rela- 

 tion to the annuli, in the hope that this may be of some assistance 

 to future workers in the field of metamerism. 



The 1 5th, :6th, and i/th segments were taken as typical in 

 this species (Hirndo medicinalis]. These segments are in that 

 region of the body where the original position of parts has been 

 least modified by displacement, fusion or otherwise. 



Gross dissections were first carefully studied, and then the more 

 exact location of organs was determined from a study of thick 

 sections, both longitudinal and sagittal. The leeches were killed 

 in about 0.05 of one per cent, solution of chromic acid, in which 

 they died in an expanded condition. Immediately after opening 

 them by a dorsal median cut through the body- wall, the latter 

 was pinned back on each side, or else two lateral cuts were made 

 and the entire dorsal portion of the body-wall removed. In the 

 latter case one could better observe the relation of the organs to 

 the sensory ring which had been previously notched at each side. 

 In preparations intended to show merely the alimentary canal, 

 the blood, which is usually found to distend it, served, when 

 hardened by the reagents used, to keep the canal distended and 

 thus to preserve its shape. 



In case organs lying below the alimentary canal, such as nerve 

 cord, testes, etc., were to be studied, the canal was at once 

 opened dorsally and the blood washed out. The preparation 

 was then kept for a few hours in water, and the organs to be 

 studied were exposed by the removal of connective tissue. 

 Extreme care was necessary in order that the organs should not 



161 



