No. 3] DESCRIPTION OF CLEPSINE PLANA. 413 



This ring, then, corresponds to the 4th and 5th rings of C. 

 chelydrcB. 



The 2d ring (6th and 7th of C. cJielydrcB) is also double, 

 as shown by its width and by a slight division, sometimes notice- 

 able at the margins. This ring represents the third ring of the 

 second somite, united with the first ring of the third somite, as 

 is plainly shown by the sense-organs being placed in the poste- 

 rior half of the ring. 



The 3d and 4th rings are of equal width, and slightly nar- 

 rower than the double rings preceding them. These, together 

 with the posterior and sensory half of the 2d ring, constitute 

 the third somite. 



The 5th (sensory) and 6th rings (buccals) are a trifle wider, 

 distinct above, but united below except at the margins. These 

 two rings form the posterior limit of the head, and together 

 form the first ring behind the suctorial surface of the ventral 

 side. When the leech is at rest, with the head attached, a 

 feeble constriction may usually be seen, which falls between 

 the 6th and 7th rings, and thus obscurely marks off the head. 



The 7th and 8th rings, the post-buccals, are distinct above, 

 but consolidated below. From this point onward the rings are 

 regular and distinct, both above and below, until we reach the 

 twenty-third somite. 



In the head we have found only two incomplete somites (ist 

 and 2d), i.e. somites with less than three distinct rings. The 

 twenty somites, from III to XXII inclusive, are complete in 

 respect to the number of constituent rings. The body termi- 

 nates behind with four short and incomplete somites (XXIII- 

 XXVI). The twenty-third has two distinct rings (62d and 

 63d) ; the twenty-fourth, one plainly double ring (64th), consist- 

 ing of a sensory ring and a narrow, imperfectly defined rudi- 

 ment ; the twenty-fifth, one ring (65th), giving evidence of its 

 double nature only at the margins ; and the twenth-sixth, one 

 narrow and simple sensory ring (66th). Adding seven somites 

 for the disc to the twenty-six pre-anal somites, we have as the 

 whole number thirty-three, which agrees with the number of 

 ganglia in the ventral chain. 



In regard to the abbreviated somites, it will be noticed that 

 we have here, as in all the other Hirudinea, the greatest reduc- 

 tion in the number of rings in the two end somites. Reduction, 



