TRANSMIGRATIONS AND METAMORPHOSES. 201 



coupled distome, that is to say, by the side of each large 

 and fat individual, full of eggs, there is one which is 

 slender. It is the Distomum filicolle, to which the 

 name of Monostomum was at first given. We should be 

 correct in supposing that of these two hermaphrodite 

 worms one acts rather as a female, the other as a male. 

 It is doubtless in this sense that Steenstrup maintained 

 his assertion, that there are in nature no hermaphrodites. 



Thus there are two kinds of distomes : the first live 

 in couples in a cyst, the second in couples joined 

 together, but at liberty; and in each case only one 

 individual produces eggs. These are distomes which 

 act really like dioecious worms. We find, however, a 

 more remarkable instance in the Monostomum bijugum 

 of Miescher. In the tumours which are formed in the 

 beak of the grosbeak (Fringilla) , he has constantly 

 found two individuals ; and in many cases he has sur- 

 prised them, with the penis of one engaged in the sexual 

 organ of its companion. These worms, while they live 

 in couples, resemble each other like snails and leeches ; 

 they are mutually fecundated, and both lay eggs. 



Leuckart recognized these sexual distomes in their 

 cyst, in the larvae of ephemerides ; and Linstow noticed a 

 distome thus sexual and encysted in the Gammarus pulex. 



The name of Monostoma has been given to some of 

 these trematodes which have no abdominal sucker. 



One of the most curious worms of this group is the 

 Monostomum mutabile. It lives in the sub-orbitary sinus 

 of several aquatic birds; that is to say, in the nasal 

 fosssB, especially of water-rails and moorhens. We 

 give a slightly magnified representation of them. It is 

 a worm resembling an elongated leaf. By compressing 



