ANATOMY OF ARENTCOLA ASSTATHJS. 759 



The situation and structure of the nerve-cord agree with the 

 description given on p. 750. Sections takeii in the niid- 

 hrnnchial region show one, two, or sometimes three giant- 

 libres. Serial sections of three segments (fig. 11) of tliis part 

 show that in tlie first and hast seo-ments there are two iriant- 

 cells, and in the naiddle one only one cell. When two cells 

 are present they lie, as in A. grubii, one behind the other, 

 the anterior one being only a little distance posterior to the 

 parapodiiun. The cells are laterally situated, pyriform in 

 shape, and their single process is directed into the adjacent 

 fibrous portion of the cord. 



Sense Organs. — The nuchal organ and the reddish-brown 

 eyofi have the nsnal structure and position. 



The otocysts are somewhat smaller and lie more laterally 

 than in A. assimilis. They are almost spherical sacs 

 (fig. 15) about 0'21 mm. in diameter, which communicate Avith 

 the exterior by a tube, whose external aperture occupies a 

 similar position to the corresponding opening in A. marina. 

 It is situated near the metastomial groove, but further from 

 the brain than in A. assimilis. The otoliths are of purely 

 external origin. They consist of numerous irregular bodies 

 (quartz-grains, fragments of spicules, etc.), without any of 

 the chitinoid covering which is usually associated with the 

 otoliths of A. marina, and which forms the major portion 

 of each otolith of A. assimilis. There ai'e in each otocyst 

 from twenty to fifty moderately large bodies, the largest 

 being 55 n long and 27 /li broad, and also a quantity of finer 

 debris of similar origin and character. The lumen of the 

 tube is slit-like, and about halfway down one of the tubes 

 there are at two points large foreign bodies. In the structure 

 of its wall the otocyst agrees with that of A. assimilis 

 (p. 753). 



