Cilia. 83 



Important, however, as cilia are, in adult stages of the animals 

 referred to, they are not of inferior importance to the newly hatched 

 young. Sponges, starfishes, zoophytes, jellyfish, worms, etc., pass 

 through a ciliated larval condition, with rare exceptions, and the cilia 

 as in the active Infusorians aid in progressive locomotion. 



It might appear that in the highest animals, provided with special 

 limbs for locomotion and with complicated respiratory and digestive 

 organs, there is no necessity for cilia. It is not so. They are often of 

 importance in the Vertebrates, although sometimes they may be 

 found to persist, when the necessity for them has apparently gone. 

 Thus Amphioxus, the lowest of fishes, possesses a ciliated skin, in the 

 young condition. Larval lampreys, too, exhibit delicate hairs upon the 

 external integument, a remnant no doubt of the ciliated condition, 

 though the hairs are now rigid. The usefulness of such cilia and brist- 

 les is difficult to discover. Similarly, the cilia which line the gullet 

 in newly-hatched fishes, such as the haddock, have no doubt merely an 

 ancestral meaning. No food passes down the gullet, for the creature is 

 mouthless and subsists by absorbing the contents of the yolk-sac. The 

 cilia soon disappear, though in many Vertebrates, such as the the frog, 

 the mouth, throat, air-passages, stomach, etc., are ciliated through 

 life 1 . Nor are they absent from the highest animals, but even in man, 

 they occur in the nasal passages, the respiratory tubes, certain auditory 

 canals, the secretory ducts in the tongue and many organs, the ovarian 

 passages, and other cavities ; but their use now appears mainly to be 

 the expulsion of matters hurtful to the sensitive epithelial surfaces 

 referred to. The central canal in the human spinal cord is lined by 

 ciliated cells in childhood, but these cells are obliterated later. We 

 thus see how important is the part played by these minute and insignifi- 

 cant organs. They are efficient for locomotion, they aid in securing 

 food, they assist in excretion, they act protectively by driving hurtful 

 matters away. 



It remains to briefly describe cilia and their mode of action. A 

 cilium is simply a thread like continuation of the protoplasmic cell to 

 which it is attached. Its base, under the highest microscopic powers, 

 differs optically from the tip ; but practically the cilium is merely a 



