478 ZOOLOGY 



face of the ventral cord. G-iant nerve- cells occur in some forms in 

 certain regions. Small ganglia are found frequently in various 

 peripheral parts, more especially at the bases of cirri or of 

 sensory papillae. 



The organs of special sense are eyes, tentacles and cirri, nuchal 

 organs, and otocysts. Eyes, absent in the Oligocheeta with a few 

 exceptions and in some of the tube-forming Polychaeta as well as 

 in a few free forms of that sub-class, are very general in their 

 occurrence. Their structure is, as a rule, very simple, but in some 

 forms reaches quite a high grade of development. Usually they are 

 confined to the prostomium, but Polyophthalmus, in addition to the 

 prostomial eyes, has pairs of eye-like organs on many of the seg- 

 ments of the body. Leptochone has a pair on each segment, arid in 

 Fabricia there is a pair on the anal segment ; while in many species 

 of lobelia and all the species of Dasychonc there are eyes or eye- 

 like organs on the branchial filaments. 



Most usually the eye is (as in Nereis, p. 447 , Fig. 353) a 

 spherical capsule with a wall composed of a single layer of cells, 

 which are elongated on the inner side, i.e. the side turned towards 

 the brain, while on the outer side they are usually flattened. The 

 outer thin part of the Avail of the capsule, or cornea, is some- 

 times united with the epidermis ; when the two layers remain 

 distinct, the outer one is the outer cornea, the inner the inner 

 cornea. In many cases a thickening of the surface cuticle over the 

 cornea forms a cuticular lens. The cells of the inner portion of 

 the wall of the capsule form the elements of the retina ; they are 

 long narrow cells, sometimes composed of three distinct segments 

 (1) a clear rod, directed towards the central cavity ; (2) a middle 

 segment which is densely pigmented ; and (3) a segment contain- 

 ing the nucleus of the cell and directed towards the brain or the 

 optic ganglion, with which it is connected by a nerve-fibre. Fre- 

 quently the second and third segments are not to be separately 

 recognised, the whole of that part of the cell which contains the 

 nucleus being densely pigmented. A refractive mass fills the 

 interior of the capsule, and is sometimes distinguishable into a 

 firmer outer part, the lens, and a more fluid inner part, the vitreous 

 body. This refractive mass is often continuous with the cuticle 

 externally, and internally may be in continuity with the rods. In 

 some cases the structure of the eye is very much simpler. The 

 eyes on the branchial filaments of many tube-*forming Polychseta 

 consist each of a group of retinal cells having its own lens-like 

 body and is quite independent of the others ; the eye is thus 

 a compound one. 



Nuchal organs (Fig. 371, B,s) are very general in the Polychseta. 

 They consist of a pair of special ciliated areas or pits on the 

 posterior part of the prostomium, eversible in certain cases. 



Otocysts are only exceptionally present. They consist of capsules 



