THE CEYLON PEARL-OYSTEK. 



325 



of hy})o.straciim at one side, which, uulike 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 

 hyposti'acum 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 eflect 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 eaily stage in such a muscle- 

 pearl, with a small cyst-like hypostracum- 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 trabeculse 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 PI. 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 (n.). In the neigh- 

 bourhood of this pearl is another, not figured here, the centre of 

 which was comparable to the pearl shown on PI. XXXV. 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 

 liypostracuan-pearl associated with them, are not all of similar 

 origin. 



PI. 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, 



