ANATOMY AND CLASSIFICATION OF THE ARENICOLID^. 515 



Histology of the Nephridia. — There is a close simi- 

 larity between the different species of Arenicola in refer- 

 ence to the histology of the nephridia. The lips of the 

 nephrostome are lined by a single layer of ciliated columnar 

 cells. In young specimens the cells are smallj and the 

 greater part of each cell is occupied by the deeply stain- 

 ing nucleus, but in older specimens in which the cells are 

 much larger, the nucleus occupies only a small portion of 

 the cell. Immediately within the mouth of the funnel of 

 A. ecaudata and A. Grubii there are numerous long ridges 

 or corrugations, which converge towards the entrance to the 

 secreting part of the nephridium. This region of the funnel 

 is very richly ciliated, and in fact, if a living nephridium 

 be examined in sea-water on a slide, the ciliary action is 

 most obvious in this portion of the organ. 



There is a moderately sharp line of demarcation between 

 the cells of the funnel, which are columnar, sti-ongly ciliated, 

 and have no concretions ; and the cubical columnar or pear- 

 shaped cells of the secretory portion of the nephridium, which 

 even in young specimens contain some excretory granules. 



Sections of young specimens show that the cells lining the 

 middle portion of the nephridium are ciliated on their inner 

 ends, and their somewhat vacuolated protoplasm contains 

 only comparatively few concretions. 



In old specimens, however, concretions are very numerous, 

 and in the nephridia of a full-grown specimen of A. ecaudata 

 which we have cut into sections the concretions are of two 

 different kinds (PI. 26, fig. 49). The columnar or pear- 

 shaped cells which line the secretory portion of these ne- 

 phridia are about 20^ to 30 ^ in length. The distal portion 

 of the cell which projects into the nephridial cavity is quite 

 clear, nothing but the very thin cell-wall being visible, looking 

 almost like a " collar." The protoplasm of the middle por- 

 tion of the cell (for a distance of about 10 ju) contains 

 numerous minute, more or less spherical granules, which stain 

 quite black with iron-htematoxylin ; while in the protoplasm of 

 the basal portion of the cell there is often a considerable mass 



TOL. 43, PART 3. NEW SEUIKS. N N 



