220 Proceediiig^ of the Royal Society of Victoria. 



The Segmental Se?ise Organs. (Fig. 5). 



The segmental sense organs occur on the body and head on 

 every fourth annulus. As before stated, there are twelve situated 

 at equal distances from each other round the first annulus of each 

 segment. These organs are covered externally by the thin 

 cuticle continuous over the whole body. Each consists of a group 

 of epidermic cells longer than those of the rest of the surface, and 

 leaning towards one another. At the base of these are four or more 

 large clear cells, such as have been described by Whitman — each 

 with a distinct nucleus lying in the protoplasm and also a cres- 

 cent shaped vacuole (C.C.I). At the point where the protoplasm 

 projects into the vacuole it appears denser and stains more deeply. 

 It is possible that this may be the terminal ending of a nerve, 

 though I have been unable to trace any definite connection with 

 the clear cell. With this may be compared W^hitman's descrip- 

 tion of the clear cells, in which he observes "a small oval area 

 though the outline of this is not very sharp," and tliis urea he 

 suggests may be the nerve ending, although he cannot prove it. 

 In the very centre are the elongated sense cells which form 

 the important part of the organ. At the anterior end of the 

 body some of these cells have become so drawn out as to 

 resemble the axial fibres of the eye. I have not been able to 

 ti'ace any decided comnmnication, however, between these central 

 cells and the nerve supplying the organ, though undoubtedly 

 there must be a connection — which in the case of Hirudo and 

 Uieuiadipsa, Whitman states he could distinctly see. 



The organs are richly supplied with blood vessels, a branch 

 usually running out at right angles to the surface of the animal 

 and di\iding so as to send branches all round the organ. 



At the inner side of the clear cells are the circular muscles of 

 the body wall. The nerve supply is derived from the ganglion of 

 the segment to which each organ belongs. 



The Eyes. (Figs. 6 and 7). 



As already stated there are five pairs of eyes situated on the 

 1st, 2nd, 3rd, 4th and 7th annuli. With regard to structure 

 there is little that is new to be said. As in the descriptions 



