ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
179 
years ago. Proteid material produced by the mollusc is decomposed by 
bacteria, forming on the one hand carbon dioxide and ammonia, and on 
the other hand conchiolin. Dissolved calcium sulphate, chloride, &c., 
are precipitated as calcium carbonate, which is disposed in various ways 
in the conchiolin. The fresh conchiolin suffers oxidation, which is 
accompanied by brown coloration, and carbon dioxide is probably given 
off. Neither the appearance of the brown pigment, nor the making of 
the shell, is in the strict sense a vital process. Steinmann alludes to the 
corroboration of his work by Murray and Irvine.* 
Formation of Pearls.f — M. Leon Diguet has a note on the difference 
in origin between the true pearls and the so-called nacreous pearls. The 
latter arise from deposits of nacre formed by the secretion of the mantle 
glands round foreign particles, and are in origin analogous to the deposits 
which increase the shell. The true pearl, on the other hand, is the 
result of a special physiological process having for its object the elimi- 
nation from the body of a parasite or some special cause of irritation. 
It originates as an ampulla, filled with an organic fluid, which gradually 
undergoes condensation ; when the condensation is completed, the mass, 
which now consists of a substance analogous to conchiolin, becomes 
stratified, leaving interstices between the strata, which are filled by a 
calcareous deposit. After calcification the wall of the ampulla is so 
thin that the pearl can be readily freed by its rupture. 
a. Cephalopoda. 
Tentacles of Nautilus pompilius.J — Mr. L. E. Griffin publishes 
some notes on these structures based on the study of a large series of 
specimens. He finds that the digital tentacles have their surface 
marked by a series of annular grooves, and that the central nerve ot 
the tentacle bears swellings corresponding to the segments between the 
annulations. The inner surface of the segments is flattened, and the 
arrangement of the muscles enables them to act as suckers. Though 
the adhesive power of each particular segment is slight, their number 
gives considerable adhesive power to the tentacles as a whole. The 
author believes that these simple suckers are the homologues of the 
suckers of the Dibranchiates, and that the arms of the latter are homo- 
logous with the tentacles of Nautilus. As the suckers became more 
specialised, the tentacles became greatly reduced in number. The ocular 
tentacles of Nautilus are to be regarded as modified and displaced 
digital tentacles. The organ known as Van der Hoeven’s organ, which 
is present in the male only, is the homologue of the inferior labial 
lobe of the female, but its function is doubtful. The hectocotylus of 
the male consists of a group of four modified tentacles usually situated 
on the left side. It may however occur on the right side, or rarely on 
both sides. It seems possible that the organ was originally always 
paired. 
* Proc. Roy. Soc. Edin., 1889. 
t Comptes Rendus, cxxviii. (1899) pp. 1589-91. 
X Johns Hopkins Univ. Circ., 1898, pp. 11-12. See also Ann. and Mag. Nat. 
Hist., iii. (1899) pp. 170-6. 
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