PROTRACTILE MOUTHS 



tiny food particles which the water carries along as it 

 passes through. Many of these particles are micro- 

 scopic floating creatures which, stuck and unable to 

 get free, are captured. But this net is not absolutely 

 fixed; the vibratile filaments which cause the water 

 to circulate also carry its meshes towards the end of 

 the gill where there is an orifice leading to the stomach. 

 The meshes, carrying their tiny victims, direct the 

 prey along the way to the stomach where they are 

 digested, and other meshes take their place at the 

 other extremity of the gill, so that this extraordinary 

 net is always ready and in position, agglutinating and 

 moving at the same time. With its assistance the 

 sea-squirt catches in its own body the prey upon which 

 it can feed, using in its own way this remarkable internal 

 trap which would make any poacher green with envy. 



These animals exhibit a remarkable conformation. 

 It foreshadows, in an elementary way but still quite 

 clearly, some structures which the vertebrates show in 

 a fuller degree, and in greater complexity. They have 

 the fore-part of the digestive tube serving also for 

 respiration, a pharynx converted into a sieve-like gill as 

 in the lancelet, an adhesive mucus comparable to sticky 

 saliva. But the functional action, which enables the 

 creature to keep alive, is even more extraordinary: 

 the bringing into the body of a considerable mass of 

 water drawn from outside, the continual renewal of 

 this water, and its use as a carrier of food. Such a 

 system is indeed surprising. Nature, in her effort to 

 maintain life, utilises the strangest methods and yet 

 succeeds in attaining her end. 



These sea-squirts, eaters-of-tiny-scraps, feed upon 

 microscopic prey. They are not alone in so doing. 

 The bivalved Lamellibranchiate molluscs do the same 

 thing: they feed upon little creatures which the water 

 brings. But in their case the mechanism is different. 

 Mucus-secreting lips, which surround the mouth, play 

 an important part in securing it. The result is the 



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