GENERAL SUMMARY AND RECOMMENDATIONS. 



117 



across, and three regions are distinctly visible in the shell, the structureless, clear, 

 embryonic shell or prodissoconch marked by very regular and delicate lines of growth, 

 and forming the hinge (with 5 anterior and 5 posterior teeth, separated by a slight 

 median interspace, figs. 28, 29, on each valve) and the umbo ; the now very extensive 

 prismatic part extending to the free margin all round and having very much the 

 shape of the adult ; and finally an intermediate region which in addition to the 

 prismatic part has a lining of nacre. These three regions of the shell are still seen in 

 fig. 27, but the prodissoconch is now becoming imbedded in the later formed shell 

 and so loses its distinctness ; its umbo, however, is still prominent. This specimen, 

 which measures 1 *5 millims. in diameter, is seen from the left side ; the preceding 

 figures were seen from the right. In fig. 27 pigment has commenced to form in the 

 prismatic layer, producing 4 to 6 yellow or ruddy-brown radial bands, most marked 

 at the periphery and dying away internally. These and still larger young oysters 

 are shown in fig. 33, natural size, attached to an Alga ; while a sample of " false 

 spat " is shown in fig. 34, and enlarged in fig. 30 (Avicula vexillum, REEVE). 



The spat in all these stages of gro\vth is very actively locomotive. Although it 

 can fix itself by the byssus threads, it does not usually remain fixed for long. When 

 moving, it pulls itself along by means of the large mobile foot (see fig. 7). We have 

 many observations showing the rapidity with which it can detach and 

 re- attach itself, and the rate at which it can travel (see Part I., p. 68). 

 There is no doubt, then, that in this young stage the pearl oyster can 

 leave the weed to which it first becomes fixed and transfer its attachment 

 to a coral or nullipore fragment on the paar, or can move from an 

 unsuitable spot in search of a better. Its tendency to climb upwards 

 whenever shaken on to the floor of an aquarium (see " Narrative," Part I., 

 p. 69) is probably an indication of an instinct to ascend any solid objects 

 on the sea-bottom, which must often save it from being smothered in the loose sand. 



Our experiments on the pearl banks in 1902, and at the Galle laboratory, have 

 shown that not only the young but also the adult pearl oyster is able to cast off its 

 old attachment, move to a new place, and there spin a new byssus, and this not once 

 or twice, but repeatedly, up to eight times in fourteen days, as our records show (see 

 fig. 8). 



1 



...I 



Fig. 7. 



Fig. 8. Diagram showing the eight successive positions in which a pearl oyster formed new byssus 



strands in a fortnight. One half natural size. 



