HIRUDINEA. 



present state of our knowledge cannot 

 be admitted. Externally the body is 

 marked by a number of transverse 

 rings the annuli which are short, 

 and may be more or less indistinct. 

 These rings do not correspond with 

 the internal segments, which may be 

 indicated by incomplete partitions; but 

 they constitute much shorter portions 

 of the body, three (Branchellion, 

 Clepsine), six (lehthyobdella, Callio- 

 l>della, Pontobdetta), five (Hirudo), 

 twelve (Piscicola) of them corre- 

 sponding to one internal segment. 

 Towards the two ends of the body the 

 segments are reduced in the number 

 of rings, and in other respects. 



There is always a posterior sucker, 

 into the constitution of which a num- 

 ber of true segments (six or seven) 

 enter, and generally an anterior sucker 

 at the anterior end of the body ; both 

 are ventral. The anus opens dorsal to 

 the posterior sucker, and the mouth is 

 on the ventral surface in the anterior 

 sucker. The animal moves in a looping 

 manner by means of its suckers. There 

 are no parapodia, or setae (except in 

 Acanthobdetta), or head appendages, 

 but in one genus (Branchellion) there 

 are leaf-like branchial processes on the 

 middle part of the body. 



On the annuli in transverse rows 

 are arranged the so-called segmental 

 sense-organs or sensory papillae. The 

 arrangement of these differs on the 

 different annuli of a segment, but is 

 the same for corresponding annuli of 

 different segments. On the head some 

 of these organs are modified into eyes 

 and into the cup-shaped organs. The 



2 L 



513 



FIG. 415. Hirudo medicinalis show- 

 ing the segments according to 

 Whitman, and illustrating the 

 enumeration adopted in the text 

 (after Whitman, from Lang)., The 

 5 pairs of eyes are marked by the 

 larger, the segmental sense-organs 

 by the smaller black dots ; the 

 openings of the nephridia by ob- 

 long marks (lp-17p). The rings are 

 counted on the right, the segments 

 on the left. 30/3 1 marks the position 

 of the male, 35J36 of the female 

 genital opening, an anus ; 6s pos- 

 terior sucker which is supposed to 

 consist of 7 segments fused together. 



