260 ANIMAL PARASITES AND MESSMATES. 



They bear the name of Diplozoon paradoxum, and are 

 always double, that is to say, always united like Siamese 

 twins, being organically fastened together; they leave 

 the egg, like their congeners, isolated and hermaphrodite, 

 instal themselves separately on their host, and a little 

 time after their choice of a resting-place, they unite so 

 that the tissues, I was about to say the organs, are 

 welded to each other. They cross like two strokes of an 

 x. It is in this position that they live and die, after 

 having produced large and beautiful eggs provided with 

 a very long cable. These eggs are laid separately, and 

 attached to the gills of the fishes which give them 

 shelter. At the end of a fortnight the ciliated embryo 

 comes forth, being provided with two eyes, and seeks to 

 establish itself on a fresh host. 



Under the form of Diporpa it has a ventral sucker, 

 and a small papilla on its back, and the two individuals 

 are attached to each other cross-wise by the sucker and 

 the papilla. Notwithstanding what Humboldt says in 

 his " Cosmos," the Diplozoon is not an animal with two 

 heads and two caudal extremities, but is a double animal, 

 two hermaphrodite individuals united, which at first 

 have lived separately, and have become soldered to each 

 other at the period of maturity. 



We find a nematode, and consequently an animal 

 with the sexes separate, which presents the same phe- 

 nomena. The male and female are soldered together, 

 but the female alone undergoes development. It is the 

 Syngamus trachealis of Siebold. It inhabits the tracheal 

 artery of some gallinaceous fowls, and according to 

 recent experiments, it develops itself directly in the 

 tracheal artery of birds. 



