54 DR. H. LYSTER JAMESON AND DR. W, NICOLL ON 



it go ? From a consideration of Trematode development in 

 general, it was obvious that these larvse in Mytilus must have 

 passed a previous stage, as a sporocyst or redia, in some other 

 moUuscan host, and it was equally obvious that they were destined 

 to become adult in some final vertebrate host, to be sought among 

 the animals that eat the mussel. 



With regard to the earlier part of the life-history, Jameson 

 was struck by the occui'rence in Tapes decussatus, which lives 

 associated with the pearl-bearing mussels in the harbour of 

 Billiers (Morbihan), Brittany, of sporocysts containing Cercarice 

 closely resembling those in Mytilus and differing from them 

 chiefly in size. Jameson subsequently found similar sporo- 

 cysts in a different situation in the common Cockle (Cardiiwi 

 edide) on the pearl-bearing mussel-beds at Piel, Lancashire, and 

 these have since been rediscovered by Lebour (1906) and 

 Nicoll (1906). The occurrence of these sporocysts in Cardium is of 

 particular interest in view of the fact that the cockle, as a rule, 

 lives in close association with the mussel and is on that account 

 a not unlikely intermediate host for the mussel parasite. 



Herdman (1903-6) failed to find this parasite in Piel cockles, 

 and seems to have doubted Jameson's assertion that it occurred 

 there ; but we have found it over and over again in cockles both 

 from the original station at Piel, and from the cockle-bed at 

 Foulney in the same neighbourhood. 



Infection-experiments wei-e undertaken by Jameson in 1901, 

 in order to prove the transference of the parasite from Tapes to 

 Mytilus, but although he claimed success in these experiments, 

 they are still open to the objection that soine at least of the 

 mussels used were already infected before the experiment was 

 undertaken *. 



Pending the results of further experiments, there are only 

 the structural characters of the two larval forms to go upon. 

 Morphologically there is a very close agreement between the 

 cercaria in Cardium and that in Mytilus. They both agree in 

 having the oral sucker not more than one-fifth of the body- 

 length, and in its being not more than one-third as large again 

 as the ventral sucker. In both the pharynx is comparatively 

 large. The differences which exist are small. For instance, 

 the oral sucker in the Mytilus cercaria is relatively larger than 

 that in the Cardium cercaria; the ventral sucker is lai'ger in 

 the former in proportion to the oral and it is slightly nearer the 

 posterior end of the body instead of, as might have been expected, 

 further forward. Again, the oesophagus is shorter and the 

 diverticula longer. These discrepancies, although minute, are 

 suflficient to give pause in too hastily concluding that the two 

 forms belong to one and the same species. 



With regard to the final host of this parasite, there can be little 

 doubt but that Jameson was right in supposing it to be one of the 



* [I am at present repeating these experiments, with the aid of a Government 

 (rrant from the Royal Society. — H. L. J.] 



