108 F. W. GAMBLE, M.SC, AND J. H. ASHWORTH, B.SC. 



transverse groove (bordered by the hinder edge of the prostominm), 

 which is continued inwards as a deep pit to the hinder margin of the 

 brain (PI. 5, Fig. 25). From the posterior cornua of the latter the 

 nuchal organ is innervated. 



in its paired form and under the names "Wimperorgane," "Wimper- 

 griibschen," the nuchal organ is well known in almost all families of 

 Polychsets, and a similarly placed organ is found in Sipnncnlids, 1 not 

 to mention other more distantly related groups. It is always associated 

 with the posterior lobe of the brain, and arises as a pair of pits from 

 the surface of the prostomium. Of its development in Arenicola, how- 

 ever, we have no evidence, but the two depressions in front of the 

 main part of the organ, together with the jDaired nerve-supply, point 

 to its double nature. 



The epithelium of this deep, pigmented pit (PI. IX., Fig. 21, Nu.) is 

 composed of long columnar ciliated cells, glandular cells which secrete 

 the mucus in which the cilia work, and slender sense-cells. It seems 

 probable that the whole organ is olfactory in function. 



Otocijsts. — The otocysts of Arenicola marina are a pair of flask-shaped 

 structures projecting into the body-cavity close to the outer edge of 

 the oesophageal nerve-commissures. They open externally by a couple of 

 apertures (PI. IX., Figs. 19, a and B, 07'), at that point on the "metas- 

 tomial groove " where the latter is crossed by the first groove of the 

 body following the prostomium. The body of the flask is placed at an 

 angle with the "neck," and contains the otoliths. It is lined by non- 

 ciliated columnar sense-cells and supporting cells, which are surrounded 

 by the nerve-fibres and connective-tissue fibrils, figured by Fhlers 

 ( 1 892, pi. xii.). The neck of the otocyst is made up of a columnar 

 epithelium covered with a thick cuticle, which gradually merges into 

 the epidermis of the external surface, and ciliated cells only occur in 

 its lower portion. A shoi*t nerve from the oesophageal commissure 

 supplies the otocyst. 



If the otocyst of a fresh shore lugworm be rapidly dissected out under 

 sea water and mounted, the sand-grains will be seen to execute a most 

 extraordinary movement. Each one is rotating slowly and jostling its 

 fellows, so that the whole contents of the flask are in a state of com- 

 motion. The fluid in which the otoliths move is slightly viscous, and 

 1 Ward, 'Bull. Mus. Harvard,' vol. xxi., 1891, p. 143. 



