The more complete replacements, such as the nephridia of the genital segment of Tubifex by a subsequently formed genital duct, may be compared with the succession of the nesonephros to the pronephros in vertebrates, and of the metanephros to the mesonephros in the higher vertebrates..
Caryophyllaeus mutabilis occurs in the roach and other fresh-water fish, and passes its earlier stages of development in fresh-water Oligochaets (Tubifex).
Archigetes appendiculatus lives throughout life in the coelom of Tubifex and of Limnodrilus.
is said to produce fully developed gonads, and if kept in aquaria with Tubifex, the number of infected worms steadily increases, a fact pointing to the whole cycle being passed through, without the intermediation of a vertebrate host.
Caryophyllaeus is an elongated, flattened worm provided with one extremely mobile extremity, the other being drawn out during the animal's sojourn in Tubifex into a short hexacanth tail.