THE EYES OF ANNULATA 99 



being absent. He also notes that the eyes are imbedded in the substance 

 of the brain. They are slightly flattened spheres about 20/x in diameter, 

 and consist of a lens surrounded by a large number of closely aggregated 

 pigment granules. The worms examined were found in a limited area 

 in fine-grained sand, occupying the upper third of the neap-tide range, 

 in a belt which crossed the outflow of a small stream into Karnes Bay. 



A comparison of these with other Polychaete worms indicates that, as 

 in other animals which burrow in the sand or earth, there is a tendency for 

 the eyes to degenerate and be reduced in number. Thus in Polynoid and 

 Syllid, two pairs of eyes are visible on the prostomium ; in Eunice and 

 Phyllodoce one pair ; and in Nephthys and Trophonia the eyes are absent 

 altogether (Fig. 64). 



No eyes are present in the earthworm (Lumbricus), but these worms 

 have been shown to be sensitive to bright light, the sensation being 

 perceived by the agency of large epidermal cells which are devoid of 

 pigment. 



The commensal, parasitic, and tube-forming chaetopods, as previously 

 mentioned, also have no eyes. Among the latter, however, there are 

 certain notable exceptions. Thus, Andrews ' has described in certain 

 of the tube-forming Polychseta eyes of a highly differentiated compound 

 type ; these he found on the branchiae of Potamilla, and he considered 

 that they might give some aid to the interpretation of arthropod eyes. 

 In the species P. reniformis the number of eyes may be as many as seven 

 or eight on each branchia. This animal lives in a leathery tube which 

 may be found projecting from holes in the shells of Gasteropods and from 

 bivalve shells. At the end of the tube the cephalic branchial plumes are 

 expanded in a circular series supported by radiating stems, each bearing 

 two rows of branchial filaments, all of which are directed anteriorly in the 

 fully expanded condition. In this state the eyes are on the outer or 

 posterior sides of the main stems of the branches, there being a row of 

 three to eight on each of the twenty radiating stems. Each eye appears 

 as a convex, hemispherical protuberance on the outer or convex side of 

 the main branchial stem. The diameter of one of these eyes is about 92/x, 

 but it varies much ; smaller eyes being often found towards the tip or 

 even interpolated between some of the larger ones along the stem. Their 

 colour is a uniform dark red, but in strong sunlight the reflected light is 

 golden yellow. On section the eye is seen to be covered by a smooth 

 cuticle continuous at the base with the surrounding cuticle ; beneath this 

 the cells forming the eye are likewise seen to be continuous at the base 

 with the hypoderm cells surrounding it. The cells in the centre are 



1 E. A. Andrews, "Compound Eyes of Annelids, T. Potamilla" J. Morph., 5, 

 271, 1891. 



