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 Oercaria from Mytilus, practically 

 proves the point. 



The adults agree with Levinsen's description and figure, except 

 that the genital pore (PI. XYI, 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 mm. to "55 mm. It 

 is thei'efore 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 redviction 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 CEdemia 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 temperatiu-e of the room where I worked at 

 Billiei's 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 XYI. fig. 1 1 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 ai-rangement of the cuticular spines is the same as that 

 described for the larva. The suckers and the digestive and excre- 

 tory 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 



