THE CEYLON PEARL-OYSTER. 267 
a nucleus is absent to two alternative causes. In the first case, the 
Trematode described in my paper on the Origin of Pearls in 
the P. Z.8. for 1902 (25) may migrate out of the sac, in which a 
pearl without any nucleus, or with a nucleus consisting of a few 
residual granules, may subsequently be formed. In the second 
case, in certain localities, Mytilus edulis produces pearls through 
an agency (which I am at present trying to investigate) other than 
Vermian. These pearls may have as nuclei either a few dark 
granules or nothing that can be detected without the use of much 
higher magnification than 100 diameters. One of them is shown 
at C in text-figure 33 (p. 277). 
(v.) Musele-Pearls. 
The discovery of “‘muscle-pearls” was, I believe, first announced 
at the British Association Meeting in September 1903. 
Under this name (Brit. Assoc. Report, Southport, 1903, p. 695) 
Prof. Herdman distinguishes pearls formed “around minute 
calcareous concretions, the ‘ calcospherules, which are produced 
in the tissues and form centres of irritation.” They occur “ most 
abundantly in the muscular tissue near the insertions of the 
levator and pallial muscles.” (Report, Part V. p. 27.) Herdman 
and Hornell say, speaking of muscle-pearls : 
“at seems probable that these have been formed by the 
deposition of calcareous matter around a minute calculus in 
the tissues”.... ‘The Muscle pearls when present are 
usually abundant, and when examining under the microscope 
a young pearl of this kind, im sitw, it is common to find a 
large number of minute calcareous depositions or calco- 
spherules scattered in the neighbouring tissue. It is probable 
that the muscle pearls are formed around these microscopic 
calcospherules as centres of irritation, and as these [? the1 | 
positions are invariably in our experience close to the surface 
of the muscle or the mantle, there is no difficulty in under- 
standing that there, if anywhere, ectoderm cells might 
migrate to the source of irritation and thus be responsible 
for the deposition of a pearl.” (Report V. p. 27.) 
Muscle-pearls are especially numerous in certain localities ; 
Prof. Herdman instances (Report V. pp. 30-31) that they were 
particularly numerous on the 8.E. Cheval Paar in 1902 and 19038, 
and 
“that the vigorous and healthy oysters of the Eastern Cheval 
and Periya Paar Karai produce practically all the examples of 
this class of pearls,” 
the numbers produced on other banks being insignificant. 
Mr. Hornell, speaking of an examination of a number of pearls 
attached to the shell, states that ‘ decalcification of the pearls 
attached to muscle-scars reveals no organic nuclei, whereas the 
jattached] pearls irregularly disposed have Cestode embryos as 
nuclei, exactly as ‘fine’ pearls have.” (19, p. 12.) 
[9] 
