WORMS 



193 



Branchiomma, at any rate, these structures do not seem to be essential 

 for the most characteristic responses of the worm to changes in tlie 

 intensity of hght (Millott, 1957) ; the position is therefore somewhat 

 anomalous. 



In the simple marine archiannelids, eyes of a similar 

 type are found. In Dinophilus, for example, a minute worm 

 found among alga^, two kidney-shaped pigmented eyes are 

 found on the prostomium (Hilton, 1924) (Fig. 177). 



LEECHES (hirudinea) may be provided with 

 visual organs of a simple type varying in number 

 from 2 to 10 (Hesse, 1897 ; Herter, 1932) ; they are 

 incajDable of optical imagery although highly light- 

 sensitive, but in some species may be absent. They 

 are found near the anterior extremity of the body 

 and vary considerably in their morphology, but the 

 visual cells are always of the spherical apolar type 

 with a central optic organelle (Figs. 178-9). 



Fig. 177.— The 



rchiannelid, 



DlXOPUlLUS. 



Showing the 



paired ocelli, Oc 

 (after Sheldon- 



Harmer). 



In Branchellion these organs are unicellular; in Piscicola 

 they consist of 12 cells arranged in a row surrounded by 

 pigment. In Hcemopis both unicellular and multicellular 

 ocelli are found (Fig. 179). In the common medicinal leech, 

 Hirudo medictnalis, there are segmental papilhe with a 

 sensory function on the middle ring of each of the 26 segments. Although all 

 the sense organs are serially homologous the pairs on the dorsal surface of the 

 first five segments are purely visual, constituting ten " eyes " (Fig. 90), provided 

 with a rich nerve supply to the cerebral ganglia. At the other extremity the 



Branchellion 



Hirudo 



Figs. 178 and 179. — The Eyes of Leeches. 



Fig. 178. — The head end of the medicinal leech, 

 Hirudo niedicinalis. 

 The dorsal aspect. The body is divided into 

 segments, each of which contains 5 rings 

 (annulae). In the middle ring of each segment 

 the segmental papillte have a sensory function. 

 The first 7 (and the last 3) segments have less 

 than the normal number of rings, and the first 5 

 show two paired eyes as larger black spots. 

 El to Eg, serially homologous with the sensory 

 papillae (see Figs. 89-90) (after Parker and 

 Haswell). 



S.O. VOL. I. 



Fig. 179. — Solitary and aggre- 

 gated eyes of the horse-leech, 

 Hcemopis sanguisuga (after 

 Kappers). 



