CAUSE OF BUOYANCY. 95 



less membrane, and of a pulpy substance : perhaps the 

 yellow colour may be owing to long immersion in spirits. 

 Some authors have supposed that the ball was the ovisac 

 of the animal ; and for the first few minutes, deceived by 

 the numerous included spores of, as I believe, Bacillarise, 

 I thought that this was the case ; others have supposed 

 that it consisted of some encrusting algce or other foreign 

 organism ; but it is, in reality, a most singular develop- 

 ment of the cement-tissue, which ordinarily serves 

 to attach Cirripedes by their bases to some extraneous 

 object, but here surrounding that object and the pe- 

 duncle, gives buoyancy, by its vesicular structure, to the 

 whole. The membrane of the ball falls to pieces in 

 caustic potash, differently from the chitine membrane of 

 the enclosed peduncle, and this shows that there is some 

 difference in composition from ordinary cement. The 

 ball, when cut in two, exhibits an obscure concentric 

 structure. The whole is excreted by the two cement- 

 ducts, through two rows of orifices, one on each side of 

 the surrounded portion of the peduncle ; and I actually 

 traced, in one case, the yellow pulpy substance coming 

 out of the cement-ducts. The upper apertures are in 

 gradation larger than those below them, and they stand 

 a little further apart from each other ; these are figured 

 as seen from the outside, much magnified, at PL I, 

 fig. 6d. I did not succeed in finding the cement-glands, 

 but I followed the ducts, of rather large size, running 

 for a considerable distance as usual along and within the 

 longitudinal muscles of the peduncle. Nearly opposite 

 the uppermost aperture, on each side, the duct passes 

 out through the corium, and becomes laterally attached 

 to the outer membrane of the peduncle, at which point 

 an aperture is formed (as in other cases, by some un- 

 known process), thus giving exit to the contents of the 

 duct. Beneath this upper aperture the duct runs down 

 the peduncle, between the corium and the outer mem- 

 brane, till it comes to the next aperture, to which it is 

 also attached, and so on to all the lower ones ; but I 



