Miscellaneous. 303 



site. The true course to follow consists in seeking by observation, 

 with the aid of the principle of harmonious correlations, the various 

 animals successively inhabited by the parasite, and thus establishing 

 the series of its metamorphoses. We must study the habits of the 

 definitive host, know its mode of feeding, and the fauna of the 

 localities which it frequents, if we wish to work up to the origin of 

 the parasites it harbours, and ascertain their migrations. By pro- 

 ceeding in this manner, I have been able this year to make some 

 observations which I think are interesting, and to which I now 

 desire to call the attention of naturalists ; they have enabled me to 

 ascertain some curious correlations between the conditions of existence 

 of the Helminths, mollusks, crustaceans, and birds that live on our 

 shores. 



The sea-lark (Tringa alpina), which is so common on all our 

 sandy or muddy shores, usually contains in its intestines two very 

 different Distoma. One of these belongs to the group of the armed 

 Distoma, or Echinostoma, and may probably be referred to D. lepto- 

 somum of Creplin. It is a fine species, well characterized by its 

 dimensions, which attain 0-010 metre in length and 0-001 metre 

 in breadth ; by its ventral sucker, which is greatly developed and 

 not far from the buccal sucker, by the scaly papillae which cover 

 its body, and the collar of large spines which surrounds its head. 

 The other, which is perhaps D. brachysomum, a doubtful species 

 imperfectly described by Creplin, is distinguished from the preceding 

 by its size, which does not exceed 0-001 metre, by its equal and 

 very small suckers, by its scaly penis, by the posterior part of its 

 body being short, broad, and entirely filled by the oviducts, and 

 by other characters, into the details of which I cannot here 

 enter. 



These two Distoma only become adult in the intestine of Tringa 

 alpina ; and we can easily follow their development by examining 

 with the microscope the various parts of the digestive tube of that 

 bird. To have them in the larval state and still enveloped in their 

 cysts, we have only to open the gizzard : there we find them, often 

 in considerable number, mixed with debris of all kinds and with the 

 sand required for the trituration of the food. The cysts of D. lepto- 

 somum are only 0-080 millim. in diameter, and are formed of a very 

 thin, perfectly transparent, double envelope. In their interior we 

 may distinguish a little Distomum rolled up, and still destitute of 

 genital organs, but already recognizable by its cephalic armature. 

 Those of D. brachysomum are larger, thicker, and of a strong yellow 

 colour ; they are 0-200 millim. in diameter. Their envelope is 0-012 

 millim. in thickness, and consists of two layers which are very distinct 

 in their structure, the external one being formed of fine radiated 

 canaliculi, and the internal one composed of concentric layers. The 

 parasite enclosed in this as yet possesses only rudimentary generative 

 organs ; but the form of its digestive apparatus and the proportions 

 of its suckers can leave no doubt as to its specific identity. The 



