Land-snails 



381 



Large 



with an oval mouth. It is very common on the 

 aerial portions of plants growing in the water. 

 Specimens are sometimes found with the tentacles 

 abnormally distended, a feature 

 that would be sufficient to cause 

 some enthusiastic " splitter " to 

 make a new species of it, dubbing 

 it crassicomis ! But this is really 

 a similar case to that mentioned in connection with 

 Li7)incea triincatida and the Liver Fluke in an earlier 

 chapter. In the present instance it is a fluke that is 

 destructive to birds — Distoma raacrostoinumi. Birds 

 like the blackbird, thrush, and wagtail are very partial 

 to Amber Snails, and the parasite having hereditary 

 knowledge of the fact, gets its young into aS'. 2nitris, 

 where they push their way into the tentacles and 

 thrive at the snail's expense. But the completion of 

 their life cycle must take place in the body of a verte- 

 brate ; so when the Amber Snail is eaten by the bird, 

 the Distoma is in a position to fulfil its destiny. The 

 Graceful Amber Snail (S. elegans) has a darker and 

 more slender shell, with a longer and sharper spire, 

 but it is open to doubt if it is anything more than a 

 variety of S. putris, with which it is found, 

 and, according to Rimmer, with which it 

 couples. The Small Amber Snail (*S'. ohlonga) 

 has certainly more claim to be considered dis- 

 tinct, for its form is nearer to that of Lwincva 

 and its habitat is different. The shell is 

 oblono^-oval, rather solid, brownish, and with 

 a proportionately large spire, the body-whorl being 

 equal to only two-thirds of the whole. The mouth 

 is rounder than in the other species. Its length is 



