436 PARASITES OF ANIMALS 



paradoxum), infesting the tentacles of a snail (Succinea 

 amphibia), are in reality larval forms of a fluke (Distoma 

 macrostomum) which resides in the intestinal canal of warblers 

 (Sylviada). By experiment Dr Zeller reared the Distoma in 

 question in the intestines of whitethroats (Gurruca garrula), 

 in blackcaps (0. atricapilla), and in wagtails (Motacilla). 

 Six days after transference the Cercarias acquired sexual 

 maturity. The odd thing is, that as the sporocystic Leucoch- 

 loridia resemble insect larvse, they are attacked and swallowed, 

 by the birds under delusion. It is a curious example of 

 mimetism in favour of the fluke's welfare. To this and 

 other equally brilliant scientific results helminthologists were 

 gradually led up by the earlier researches of Steenstrup and 

 Van Beneden. As remarked in my ' Entozoa/ ever since 

 Steenstrup's discovery of the fact that Cercarise found in the 

 bodies of water-snails were larval flukes, a peculiar interest has 

 attached itself to this subject. Not only were the conclusions 

 which he elicited novel in themselves, but they formed a basis 

 for the enunciation of that interesting " law of alternate genera- 

 tion " with which the famous Danish naturalist's name will ever 

 remain associated. In all essential particulars Steenstrup's 

 statements have been verified. 



By way of illustrating the phases of development through 

 which the distomes pass I cannot do better than recapitulate in 

 an abridged form the account I have previously given of 

 Distoma (Echinostoma) militare of the snipe and curlew. This 

 account is based on the investigations of Van Beneden, Von 

 Siebold, and Pagenstecher. I regret that it is out of my power 

 to reproduce the illustrations that have already appeared on 

 this head in my introductory treatise (see ' Entozoa/ figs. 5 to 

 9 inclusive). Our Echinostoma militare produces oval-shaped 

 eggs, which give birth to a free ciliated embryo, and this embryo 

 produces a sporocyst or scolex by internal budding. When 

 the sporocyst separates itself from the embryo it presents a 

 very simple appearance, but showing already a csecal digestive 

 tube. The tail end is fissured, indicating an early stage of 

 formation of caudal appendages. In the next stage we have a 

 well-developed head and body, the tail becoming strongly pro- 

 nounced. Limb-like caudal lobes project on either side, and 

 an oral sucker makes its appearance in front. This sucker 

 communicates with the oesophageal bulb and passes directly 

 into the digestive csecum, which contains a variable number of 



