SENSE-ORGANS (JILLS 



395 



of the body. A segment of an earthworm may indeed have 

 five or six distinct annulations, but it will be bounded internally 

 by two septa, and will bear only one set of chaetae externally. 

 In the leech external clues to the definition of a segment 

 were until recently wanting. They appear now to have been 

 found in the sensory organs of the skin (Figs. 201 and 202), 

 which are, according to Whitman, 1 disposed in a perfectly 

 metameric fashion. Judged by this, and also by the nephridia 

 and nerve- ganglia, the number of segments in a leech does not 

 appear to exceed twenty-six, independently of the sucker, which 

 may represent a few fused segments, seven (in the medicinal 

 leech) according to Leuckart. 



The eyes, which are so useful in the systematic arrangement 

 of the group, appear to have been evolved from these sensory 

 organs by a further exaggeration of their peculiarities. Figs. 

 202 and 203 show this point convincingly. The segmental sense 

 organ is shown in Fig. 202 ; to the outside of certain sense cells, 

 below which are a mass of ganglion cells, are 

 certain peculiar transparent cells very similar 

 to the clear cells found in the interior of the 

 eye (Fig. 203). The segmental disposal of the 

 sensory bodies and of the eyes is shown in 

 Fig. 201. 



Some Hirudinea are furnished w T ith external 

 branchiae ; this is the case with Branchellion, 

 in which genus the branchiae (Fig. 204) have 

 an arborescent form ; in Cystibranclms there are 

 a series of paired simple vesicles which take the 

 place of these more complicated respiratory 

 organs of Branchellion. The Hirudinea do 

 not, save for one exception (Acanthobdella), 

 possess chaetae ; but it must be borne in mind 

 that the Discodrilidae and the genus Anacliaeia 

 among the Oligochaeta are in the same condi- 

 tion. In Acanthobdella 2 there are two pairs of 

 chaetae upon each side of the anterior five seg- 

 ments of the body. 



Fig. 204. Branchel- 

 lion taiyedinis. 

 (From the " Regne 

 Animal.") x 1. 



According to the figure which Grube, the 



1 Quart. J. Micr. Sci. xxvi. 188G, p. 317. 



2 See Grube, " Annulaten " of Middendorffs Sibirischc Jieise, Zoology, 1851, p. 20 

 and Kowalevsky, Bull. At. St. Pctcrsb. v. June 1896. 



