x PHYLUM ANNULATA 467 



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 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. 

 Frequently 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 Polychaeta 

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

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

 compound one. 



Nuchal organs or ciliated pits (Fig. 377, B, s) are very general in 

 the Polychaeta. They consist of a pair of special ciliated areas or 

 pits on the head, eversible in certain cases. 



Otocysts (statocysts) are only exceptionally present. They consist 

 of capsules of ciliated cells, in the fluid contained in which there 

 are one or several calcareous otoliths (statoliths). 



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

 hair or stiff cilium, are very common, especially on the prostomium 

 in the Oligochseta and on the tentacles and cirri in the Polychseta. 

 Groups of these are often aggregated together in papilla 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 Chsetopoda 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, primarily 

 ectodermal in origin, 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 ccelome 

 either of the same segment as that on which the external aperture 



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