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Show 1902.] ORIGIN OF PEARLS. 159 in all directions. My discovery in the small intestine of a Billiers Scoter, three inches behind Meckel's diverticulum, of a single immature example of the parasite, positively identical in size and all details with the Cercaria from Mytilus, practically proves the point. The adults agree with Levinsen's description and figure, except that the genital pore (PL XVI. fig. 11, g.p.) is just in front, and not in the centre, of the ventral sucker. Levinsen's observation on this point has been treated with scepticism by later writers, and, indeed, such a position for the opening of the penis and other genital tubes would not only be a novelty in Trematode anatomy, but would probably render the sucker useless as an adhesive organ. The adult worm varies in size from "2 m m . to "55 m m . It is therefore only about half the size of the larva found in Mytilus. It is protandrous, and specimens in which the male organs are ripe are generally larger than egg-laden females. The diminution in size accompanying sexual maturation is of great interest, and can be accounted for in this way, I think. The resting-stage in Mytilus is a highly assimilative phase in the worm's existence. The bulk of the body is greatly increased by the distended gut and excretory system. A reduction in bulk would be effected by the discharge of the contents of the latter, but still more by the absorption of reserve material required to mature the gonads. In fact, the reproductive organs seem to grow at the expense of the other tissues of the body. The gut, I may mention, is empty in the adult worm. Specimens from QJdemia die very quickly as a result of the post-mortem cooling of the body of their host. The Scoter that I shot was still warm when I dissected it, but the parasites died in a few minutes when placed on a slide, and an hour after the bird had been opened every adult worm was dead. The immature specimen above mentioned survived the death of the host by twelve hours. The temperature of the room where I worked at Billiers was very low, and possibly in a well-heated laboratory death would not ensue so quickly. Owing to the rapid death of the worms, the material that I preserved was not suited for detailed histological work, but the relations of the various organs to one another could be determined on sections, and by this means I have been able to check my observations on pressure preparations of fresh material at Billiers. Plate X V I . fig. 11 shows the structure of an average individual in which the maximum number of eggs has not yet been reached, and the uterus is not too complicated to mask the other structures. The arrangement of the cuticular spines is the same as that described for the larva. The suckers and the digestive and excretory systems are also the same. The genital pore (g.p.) is just in front of the ventral sucker. The penis (pe.) is pyriform. Its extremity seems to be beset with glands. Posteriorly it goes over into the large sac-like seminal vesicle (s.v.). This lies dorsal to |
