HABITS AND STRUCTURE OF ARENICOLA MARINA. 7 



when the proboscis is withdrawn, is a slightly crescentic trans- 

 verse slit, bordered by papillae and somewhat overhung by an 

 upper lip. The anus, which is terminal, is often protruded, 

 and the thin vascular swollen lips of the aperture project 

 behind the last caudal segment. 



The opening of the '^ nuchal organ^' is a fairly wide slit on 

 the upper and hinder border of the prostomium (PI. 4, fig. 19, 

 A and B, NV.). Through this aperture, sea water (or a mixture 

 of sea water and the secretion of the surrounding glandular 

 cells) is probably introduced. 



The openings of the otocysts are difficult to see. They lie 

 behind the prostomium on each side of the anterior end in the 

 position marked OT. (PL 4, fig. 19, a and b). Each is placed 

 at the point of intersection of the first transverse groove 

 following the prostomium, with the oblique " metastomial '' 

 groove which marks the position of the nerve commis- 

 sure. 



The nephridial openings (fig. 1, NO), six in number on 

 each side, though not so distinct as in some species (e. g. A. 

 Claparedii), are not difficult to find. The first is placed 

 behind and at the upper edge of the fourth neuropodiura, and 

 the other five in corresponding positions on the succeeding 

 somites. They are minute slightly oblique slits, sometimes 

 exhibiting tumid lips. 



Skin. — The skin is subdivided into raised polygonal areas 

 separated by corresponding shallow grooves, and is noteworthy 

 in being devoid of special glands. Wiren (1887) has shown 

 that the grooves are composed of columnar cells containing 

 pigment granules, the raised areas being made up partly of 

 larger cells containing still greater quantities of pigment 

 granules and partly of clavate mucus-forming cells, which 

 produce the slimy covering of the animal with which the 

 burrow is lined. 



The 5 per cent, formalin solution of the epidermal pigment 

 is fluorescent, but does not yield any absorption bands, merely 

 cutting ofi* the rays at the blue end of the spectrum. In suc- 

 cessively thicker layers of this solution, first the violet, then 



