OCR Text |
Show 1902.] ORIGIN OF PEARLS. 155 small simple Sporocysts, or, at most, groups of three or four secondary ones. Examples which averaged 17-21 m m . in length contained masses of seven to ten cysts, those measuring 27 m m. had still larger groups, while from that size upwards there was a steady progression in the dimensions and number of constituent units in the groups of Sporocysts. Some of those found in Tapes 40-50 m m . long measured 7 m m . in diameter, and contained as many as thirty secondary cysts, and a hundred Cercariae or even more. As the Sporocyst grows, it sometimes compresses the tissues that intervene between it and the shell, which apparently interferes with the secretion of fresh shell-layers. This leads to the development of white chalky spots on the inner surface of the valve. These patches, in old individuals, further prove that the large groups of Sporocysts are the descendants of the original small ones which are present when the molluscs are young. For we may see two or more white scars on the lining of the shell, marking the position of the cysts when the shell was younger. In fact, the Sporocysts may leave, imprinted on the shell, the history of their movements as the mantle-margin shifts outwards, just as the adductor muscles mark on the nacre the record of their migrations. The first or innermost of these scars is a small one, such as might result from a triple or quadruple cyst, the next is larger, while the group of Sporocysts in the mantle-margin is larger again. This plainly shows that as the mantle-margin followed the peripheral growth of the shell, the group of Sporocysts increased in size. These compound Sporocysts are, of course, thicker than the normal thickness of the mantle, and stand out as opaque white granular eminences, obvious as soon as the shell is opened. On the beds of Pearl-bearing Mussels in the Barrow Channel, opposite the Piel Fish-Hatchery, where every specimen of Mytilus is abundantly infected with the Leucithodendrium, and almost every specimen contains pearls, Tapes is not found. The Cockle, Cardium edide L., is common there, and acts as a host for the Sporocysts. Somewhat less than half the specimens of Cardium that I examined at Piel were infected. In Cardium the Sporocysts occurred in the mantle-margin, close to the anterior border of the anterior adductor muscle. Large groups, such as occur in Tapes, were not observed, but only single, triple, or quadruple cysts. . „ I have not yet been able to trace the infection of Tapes or Cardium. It is therefore impossible to say whether infection takes place by means of a free swimming Miracidium larva or not. The constant occurrence of the Sporocysts in exactly the same positions suggests that the eggs are carried into the digestive system of Tapes with the food-bearing current, hatch out m the alimentary canal, enter the circulatory system, and reach their destination via the posterior pallia! artery, along the course of which they are distributed in Tapes. Moreover, the position in |