THE CEYLON PEARL-OYSTER. 325 
of hypostracum at one side, which, unlike the nacre, does not 
increase appreciably in thickness. The centre of each is a cavity, 
which in figs. 8 & 10 is obviously lined with hypostracum, and this 
hypostracum is connected by a plug of the same substance with the 
remaining muscle-attachment epithelium. In fig. 9 the growth 
of the nacre has pulled down the plug of muscle-attachment 
epithelium into the hilum, and produced quite a long strand of 
hypostracum-like substance. The presence of these hila, together 
with the effect of the mutual pressure of muscle-pearls when 
crowded together, has much to do with the generally irregular 
shape of commercial seed-pearls. Fig. 10, also from one of 
Prof. Herdman’s slides, shows a very early stage in such a muscle- 
pearl, with a small cyst-lke hypestracum- pearl alongside it. Here 
the ordinary epithelium of the sac seems to be gaining on the 
muscle-attachment epithelium. 
These muscle-pearls always contain a central cavity, which may 
be broken up by trabecule of hypostracum-like substance or of 
conchyolin, this substance being continuous with that forming 
the lining of the cavity. The cavity, like that of the pure 
‘hypostracum-pearl, may be empty or may contain more or less 
granular matter. 
Muscle-pearls are often clustered and may be very numerous. 
Thus the old unlabelled material in the British Museum has 
dense clusters of these pearls in some places, and so has some of 
Dr. Kelaart’s material. 
It is by no means the case that muscle-attachment epithelium 
always persists in the sac of a muscle-pearl. The whole sac 
may pass over at an early stage into nacre-secreting epithelium, 
a process which is, perhaps, analogous to what occurs in the wake 
of an advancing muscle in the growing shell. This was apparently 
the case with the pearl that occupied the sac adjoining the body 
shown on Pl. XXXVI. fig. 11. The nucleus of this pearl is 
shown at fig. 12 on the same plate. The central portion of this 
pearl is composed of irregular conchyolin-like substance, which 
cannot be identified as hypostracum, and which quickly 
gives place to ordinary nacreous substance (7.). In the neigh- 
bourhood of this pearl is another, not figured here, the centre of 
which was comparable to the pearl shown on Pl. XX XV. fig. 8; 
this pearl had become more spherical secondarily by the dis- 
appearance, in the course of its growth, of the muscle-attachment 
epithelium. It is hard to conceive that these two pearls, and the 
hypostracum-pearl associated with them, are not all of similar 
origin. 
Pl. XXXVI. fig. 13 shows the centre of another pearl, perhaps 
a muscle-pearl, from the mantle-margin, in one of Prof. Herdman’s 
slides. This pearl appears to have measured about 2 mm. in 
diameter. The central cavity is about 0:1 mm. in its greatest 
diameter and is lined by abnormally thick conchyolin-like 
substance. Outside this are layers of ordinary nacre, which pass 
over into a form of repair-nacre showing radial markings, 
[67] 
