No. I.] THE METAMERISM OF NEPHELIS. 23 



I attempted to separate all the individuals collected from one 

 locality near Worcester, Mass., according to color and color 

 marks. I provided five aquaria and sorted each lot as I col- 

 lected them until the whole number of individuals exceeded one 

 hundred and twenty-five. It was very evident at a glance that 

 the leeches in the first aquarium were light colored, and that 

 those in the fifth were dark colored, but it was impossible to 

 divide them so that each aquarium should be free from tran- 

 sitional forms. I repeated the effort on my collections from 

 Wolf Lake, near Chicago, III., and with like results. The very 

 light and very dark individuals were about equally rare, while 

 the great bulk of each lot was made up of leeches varying in 

 shade but having the same stripes more or less distinctly 

 accented according to the amount of pigment present. These 

 trials led me to adopt the method proposed by Whitman (7) and 

 used by Blanchard (4). The method consists in determining 

 the number of rings in the entire body and the limits of each 

 somite. The first ring of each somite in the Hirudinea bears 

 eight sense organs on the dorsal side, as Whitman has shown, 

 four of which are serially homologous with the eyes. 



The typical somite of Hirudo contains five rings. This 

 number holds good throughout the middle body region, but 

 falls to three towards the two ends, then to two, and finally to 

 one. The amount and the manner of reduction vary in dif- 

 ferent genera, but are constant in any given genus. In Nephe- 

 lis, also, the typical somite has five rings, but the limits of 

 the somites and the number of rings in the terminal ones are 

 not readily determined by the arrangement of the sensillae, for 

 with certain exceptions mentioned hereafter these appear about 

 equally prominent on every ring throughout the entire body. 

 My first attempts to determine these points by means of the 

 sensillae failed ; I succeeded later in the following way. 



When Nephelis is thrown into weak chromic acid, — Yd to 

 Yi per cent solution, — there soon comes a time when the 

 sensillae stand out with perfect distinctness ; later the contrast 

 in color between them and the surrounding surface becomes 



' C. O. Whitman : " The External Morphology of the Leech." Proc. Am. 

 Acad, of Science^ vol. xx, p. 76, 1SS4. 



