RUDIMENTARY ORGANS OF ANIMALS 127 



Many facts in support of our argument may be 

 drawn from a study of the excretory organs of the 

 Cestodes. 



The condition of these organs in Caryophylleus 

 mutabilis best represents the primitive condition. 

 Within the body of this worm are a large number 

 of narrow ducts with ciliated funnels communicating 

 with the spaces in the parenchyma (Fraipont). 

 These organs communicate with canals which 

 gradually reunite and anastomose to find a vent in 

 one single aperture, the foramen caudale, which is 

 situated in the posterior part of the body where 

 there is a bladder. In the Cestodes, however, where 

 the body is very long, the action of the bladder 

 is insufficient to secure a complete evacuation. 

 Secondary apertures are therefore formed at in- 

 tervals along the main ducts. This new structure 

 entails the degeneration of the terminal bladder 

 which has become superfluous. In Botryocepludus 

 punctatus, which possesses a great number of 

 excretory apertures, the primitive evacuatory appa- 

 ratus i.e. the contractile cavity has completely 

 disappeared. 



Rotifers are minute animals, usually living in 

 fresh water, a few being marine. One of them, an 

 inhabitant of damp earth or moss, has been supposed 

 to possess the power of revivifying after complete 

 dessication. At the anterior end of the body, a 

 rotifer possesses a complicated ciliary apparatus 

 which fulfils the function of locomotion, and from 



