234 INVERTEBRATE MORPHOLOGY. 



longitudinal rows of papillae, six on the dorsal surface, six on 

 the ventral surface, and two marginal. In the anterior and 

 posterior segments whose width is reduced the marginal pa- 

 pillae may be wanting, but throughout the rest of the body 

 the number of rows is constant. In structure these papillae 

 are . somewhat complicated, consisting of an axial bunch of 

 elongated sensory cells bearing fine cilia at their outer ends, 

 and lying in the connective tissue in their immediate vicinity 

 is a varying number of large cells, each containing a large 

 watery vacuole in the interior, the nucleus, in consequence, 

 being pushed to one side. A strong nerve runs to each pa- 

 pilla and is supplied to the large vacuolated cells as well as 

 to the axial sensory cells. 



Slight differences are to be found in various forms in the structure of 

 these organs. In Clepsine there is an axial bunch of hair-bearing cells to 

 which the terminal fibres of the nerve run, and posteriorly and below the 

 nerve are found the large vacuolated cells. In Hirudo and Neplielis no 

 hair-bearing cells occur, the nerve occupying the axis of the organ and the 

 vacuolated cells being arranged symmetrically around it. 



It is probable, in view of the two kinds of constituent elements in Clep- 

 sine, that in this and similar genera a double function is possessed by the 

 sensory papillae, the hair-bearing cells having perhaps a tactile function, 

 while the vacuolated cells are visual. It seems probable also that primarily 

 the papillae were similar in structure and function to the organs of the 

 lateral line of certain Polychaeta, such as the Capitellidae, or perhaps it would 

 be better to compare them with the tactile papillae of certain aquatic 

 Oligocha3ta, which in the genus Slavina have an arrangement on each meta- 

 mere recalling that found in the Hirudinea. 



Towards the anterior extremity of most of the Hirudinea 

 a varying number of eyes are found. In some species of 

 Clepsine but two such organs occur, while in others there are 

 six, and in Ilirudo, Macrobdella (Fig. 106, oc), and allied forms 

 there are always ten. In the latter forms the eyes are always 

 arranged in a definite manner: one pair is situated on the 

 anterior ring (when more than one ring occurs) of each of the 

 five metameres immediately following the prostomial lobe, 

 and if their position be determined it will be found that they 

 occupy the place of one of the dorsal sense-papillae, the eyes 

 being serially homologous with the sense-papillae of one of 

 the dorsal rows. This conclusion is verified by their struc- 



