TREHATODA. 



321 



flavum find mutabile), which it carries about until it enters the first 

 host (fig. 256, b). In other cases the course of development is sim- 

 plified by the omission of the second intermediate host, viz., that 

 which contains the encysted immature Distomum (Cercaria macro- 

 cerca of Distomum cygnoides, also Leucochloridium in the tentacles of 

 Helix succinea). 



(1) Sub-order : Distomea. Trematodes with at most two suckers, 

 without hooks. They develop with a complicated alternation of 

 generations. The asexual individuals and the larvse live principally 

 in Mollusca, the sexually mature animals in the alimentary canal of 

 Vertebrates. 



The sexes are completely separated in 

 Distomum hcematolium (from the veins of 

 man); individuals of the two sexes being 

 united in pairs (fig. 257). Dimorphic forms 

 are found in certain species of the genera 

 Monostomum and Distomum in connection 

 with the division of labour of the sexual 

 functions ; one individual develops only male 

 sexual organs, and the other only female, the 

 former producing spermatozoa and the latter 

 ova. The rudiment of the functionless 

 generative gland undergoes in these cases a 

 more or less complete degeneration. Such 

 Distomea are morphologically hermaphrodite, 

 but practically of separate sexes. 



The complete biology and developmental 

 history is unfortunately only satisfactorily 

 known for a few species which can be fol- 

 lowed through all the stages of development. 



FiO. 257Dittomum haemato- 

 Hum. Male and female, 

 the latter being in the 

 canalis gynaecophorus of 

 the former. 8, sucker. 



Fam. Monostomidae. Of an oval, elongated, more or less rounded form, with 

 only one sucker, which surrounds the mouth. Monostomum Zeder. Sucker 

 surrounding the mouth ; pharynx powerful. Sexual openings but slightly 

 removed from the anterior end. M. mutabile Zeder, in the body cavity and in 

 the orbit of various water-birds ; viviparous. M. flavum Mehlis, in water-birds, 

 develops from Cercaria ephemera of Planorbis. M. lentis v. Nordm., the 

 young form without generative organs is found in the lens of the human eye. 

 M. bipartitum Wedl., living in pairs enclosed in a common cyst, the one indi- 

 vidual surrounded by the lobed posterior end of the other ; branchiae of Tunny- 

 fish. 



Fam. Distomidse. Body lancet-shaped, frequently spread out. more rarely elon- 

 gated and rounded with a large median sucker ; in front of which lie the genital 

 openings, usually close together. 



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