x PHYLUM ANNULATA 439 



Most usually the eye is (as in Nereis, p. 410, Fig. 324) 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 wall of the capsule or corn/, is some- 

 times united with the epidermis ; when the two layers remain 

 distinct, the outer one is the outer cornea, the inner the in<'/- 

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

 cornea forms a eutiri/lar 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 some- 

 times 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. 



Otocysts are only exceptionally present. They consist of capsules 

 of ciliated cells, in the fluid contained in which there is one or 

 several calcareous otoliths. 



Ciliated grooves occur on the prostomium of many forms ; in 

 Aricia they are present on all the segments ; they have a special 

 nerve-supply, but their function can only be conjectured. Tactile 

 cells of the epidermis, with or without a projecting tactile hair or 

 stiff cilium, are very common, especially on the prostomium in the 

 Oligochaeta and on the tentacles and cirri in the Polychseta. 

 Groups of these are often aggregated together in papillce or 

 goblet-bodies with special nerve-supply, and often with a ganglion 

 or a single nerve-cell at the base. 



The organs of excretion of the Chaetopoda are a series of 

 segmentally arranged tubes, the nephridia, of which a pair, as a 

 rule, occur in each of the segments of the body, with the exception 

 usually of a few at the anterior and a few at the posterior end. 

 In its simplest form the nephridium is a curved tube, ciliated 

 internally, opening on the exterior by a laterally placed pore at 

 the one extremity, and at the other ending in a ciliated funnel 

 or nephrostome, which opens into the cavity of the corresponding 

 segment. The nephridia thus effect a communication between 

 the coelome and the exterior, and serve to carry off waste products 

 which have passed into the ccelomic fluid ; but in many instances 



