650 THE ANIMAL KINGDOM. 



the same moment moult its ciliated coat (D. cylindraceum, (?) D. mentula- 

 tum\ but this coat is not usually lost until the embryo enters its first host. 



The monogenetic Trematoda frequent a single host and are ecto- 

 parasitic, i. e. do not as a rule inhabit the internal organs of this host. The 

 exceptions are Calicotyle, which has been found in the cloaca of Rays ; 

 Aspidogaster conchicola^ which inhabits the nephridia of the Lamellibranch 

 Anodonta^ and A. limacoides from the intestines of the Teleosteans, Leu- 

 ciscus idus and L. dobula ; Cotylaspis from the American Anodonta ; Poly- 

 stomum integerrimum from the bladder of Rana temporaries, R. esculenta 

 and Bufo viridis, and P. ocellatum from the pharynx of the Chelonian Emys 

 europaea. The other genera are found for the most part attached to the gills 

 of various fishes, or on the surface of their bodies, e. g. Tristomum Molae. 

 Udonella Caligorum lives on the various species of the Crustacean Caligus. 

 The majority are marine. P. integerrimum, Diplozoon, and Gyrodactylus 

 are severally peculiar. The first-named attaches itself as an embryo to the 

 internal gills of frog-tadpoles. When the gills atrophy at the time that the 

 tadpole changes into a frog, the young parasite migrates through the 

 digestive tract into the bladder, where it becomes sexually mature in three 

 years, but does not attain its full size for five or six years. But if the em- 

 bryo attaches itself to the gills of a very young tadpole, it undergoes a 

 premature sexual development, does not migrate, and dies when the tad- 

 pole undergoes metamorphosis (cf. note i, p. 648). Diplozoon paradoxum 

 consists of two individuals fused together. The embryo known as Diporpa 

 is at first free-swimming ; it soon loses its cilia, and settles on the gills of 

 a Minnow ; loses its eyes, but lives in a single condition for weeks or 

 months ; but finally one individual attaches itself by its ventral sucker to 

 a conical eminence on the back of a second individual, which thereupon so 

 twists itself as to fix the first individual in the same manner. The cones 

 and suckers fuse completely ; in other respects, however, the two Diporpae 

 which make up a single Diplozoon are independent of one another. Gyro- 

 dactylus elegans, found on the gills and fins of various freshwater fish, is 

 viviparous, but the embryo before it is extruded, itself contains an embryo, 

 and this in turn another, so that three generations of embryoes are repre- 

 sented simultaneously. 



The digenetic Trematoda have at least one intermediate generation, 

 but as a rule very many before the sexual organism reappears. The 

 embryo, whether it is or is not ciliated, enters first a host which is very 

 rarely some Fish, but usually a Mollusc, either a Gastropod or Lamelli- 

 branch, but not necessarily always the same Mollusc for the same species 

 of fluke l . The egg-shell with the embryo may be swallowed by a snail 

 and the embryo set free in the alimentary canal, a mode by which land 



1 Sporocysts are occasionally met with in Fish. A free swimming Sporocyst has been observed 

 by Ramsay Wright (American Naturalist, xix. 1885, p. 57). 



