156 THE VOYAGE OF H.M.S. CHALLENGER. 



projection, occupying not only the \Yliole of the peritubercular area, but extending across 

 the prebranchial zone and pushing the tentacles anteriorly (PI. XL fig. 7). The anterior 

 border of the tubercle is straight and the posterior gently curved, while both ends are 

 rounded. The projection formed by the tubercle is very considerable, so that in 

 transverse sections it forms roughly about three-fourths of a circle (PI. XL fig. 8). 

 The whole surface of this greatly enlarged dorsal tubercle is smooth, but has a finely 

 granular appearance and is soft and spongy to the touch. No traces of coiled horns, 

 of apertures, or of any other markings are visible. 



When thin transverse and longitudinal sections of the dorsal tubercle are examined 

 under a low power of the microscope ( x 50 diam.), it is seen that the entire free surface 

 is covered by columnar epithelium, which is l^roken up by the very numerous narrow 

 apertures of ciliated cavities leading into long narrow tubes embedded in the thickness 

 of the tubercle (PI. XL fig. 9). I have estimated that there must be about 50,000 of 

 these tubes and the same numljer of openings on the surface of the dorsal tubercle. 

 The tubes do not penetrate to the centre of the mass, but form a broad zone, occupying 

 nearly the outer half, which is clearly visible both to the naked eye (PI. XL fig. 8) and 

 in the microscopic specimens (PI. XL fig. 9). The rest of the tubercle is composed of a 

 mass of connective tissue continuous with that of the mantle below, and penetrated by 

 a number of blood sinuses. Where this mass of connective tissue joins the mantle 

 proper, muscle bands are found cut in various directions (PL XL fig. 9, m.h), and at one 

 side of the section is invariably found either one or two large nerves (PL XL fig. 9, n.)', 

 but no nerve fibrils were found arising from these or distributed through the outer part 

 of the organ. 



Each opening on the surface of the dorsal tubercle leads into a globular or ovate 

 cavity (the iufundibulum, PL XL fig. 10, inf.), from the opposite end of which a long 

 more or less cylindrical tube leads down into the connective tissue. The walls of 

 the infundibulum are formed of tall columnar cells which bear numerous larcce cilia. 

 At the lower end of the ciliated infundibulum there is generally a marked constriction 

 (see PL XL fig. 10) separating ofi" the glandular portion of the tube. Beyond the 

 constriction the wall of the tube is formed of a delicate layer of squamous epithelium, 

 inside which is found a layer of cubical or rounded cells with well-marked nuclei and 

 granular contents. In many of the tubes the lumen is to a great extent filled up by 

 masses and strings of these granular rounded cells (PL XL figs. 10, 11, and 12), and in 

 these cases a delicate network, formed of small cells with long much-branched processes, 

 is found extending inwards from the outer squamous layer of the wall of the tube, and 

 dividing the lumen into a number of imperfect sections or crypts in which the large 

 gland cells are packed (PL XL figs. 11 and 12). The connective tissue between and 

 around the infundibula and the tubules consists of a clear homogeneous matrix, in 

 which are embedded numerous small rounded and fusiform cells (PL XL fig. 10, c.i.). 

 Irregularly-shaped blood sinuses (hi. s.) are also present. 



