HABITS AND STIIUOTURE OF AliENICOLA MARINA. 25 



above iVw., PI. 4, fig. 19, B.). These two pits form the beginning 

 of the nuchal organ and indicate its paired origin. Further 

 back they unite to form a transverse groove (bordered by the 

 hinder edge of the prostomium), 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/^ 

 '' Wimpergriibschen/' the nuchal organ is well known in 

 almost all families of Polychsets^ and a similarly placed organ 

 is found in Sipunculids,^ 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, however^ 

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

 main part of the organ, together with the paired nerve-supply, 

 point to its double nature. 



The epithelium of this deep, pigmented pit (PL 4, 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 olfac- 

 tory in function. 



Otocysts. — The otocystsof 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. 4, figs. 19, a 

 and B, OT.), at that point on the " metastomial 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 Ehlers (1892, 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 

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



