PROTRACTILE MOUTHS 



The water enters their organ of respiration, the 

 gill-cavity, a large pouch which fills the greater part 

 of the interior of the body. The inhalent siphon, 

 which draws in the water, leads to the mouth, and the 

 mouth opens into the gill-pouch, for this is nothing 

 but an enlarged pharynx, which by its size is capable 

 of dealing with respiration and nutrition. It is pierced 

 by a large number of little openings, like a very fine 

 sieve. The water passes through these orifices and, 

 leaving the branchial or gill-pouch, is collected in 

 another pouch which surrounds the first. The gill- 

 pouch is like a cylindrical sieve contained in an 

 outer sac, and this is fitted with a spout to empty it. 

 This is the second siphon, which serves to throw out 

 the water, and is called the exhalent siphon. 



When this apparatus is in action, the water enters 

 by the first siphon into the sieve-like pouch, filters 

 through its many tiny openings into the surrounding 

 casing, and finally flows out again. By this means a 

 continual fresh current is assured. The agent which 

 forces the water to circulate continually in one way is 

 the beating movement of innumerable microscopic 

 vibratile filaments with which the edges of the openings 

 of the sieve are fitted. They all beat without ceasing, 

 sweeping the water along, passing it from the interior 

 of the inner pouch into that of the containing pouch, 

 and so setting up and maintaining the circulation. 



The interior sieve-like pouch serves a double purpose, 

 for both feeding and respiration. In the first place it 

 acts as a gill. In the framework round the holes 

 perforating it there are blood-vessels, in which the 

 blood, as it flows, gets rid of its carbonic acid and 

 absorbs a part of the oxygen dissolved in the water 

 which is continually arriving. It serves, further, as a 

 means of securing food. On its inner face, it has a 

 network of mucous strings, which form, as it were, 

 a sort of sticky net, whose meshes cover the holes of 

 the sieve in all directions. These strings catch up the 



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