PROTRACTILE MOUTHS 



alone, is to be observed even more completely in other 

 creatures, like the coral polyps, which are like the sea- 

 anemones in structure, for they are a sort of small 

 edition of the latter. They too have a large stomach, 

 which is almost always empty, showing no effective 

 trace of a digestive process at work. All are associated 

 and bound by canals which go from one to another, and 

 the water, coming from outside, circulates in all, renews 

 itself continually, penetrates their common mass, irrigates 

 it without ceasing, as in the case of the sponges. And 

 this is enough for them. This continual circulation 

 allows some of them to extract from the water the vast 

 amount of lime needed in the production of their large 

 polyparies, the heaping together of which forms islands 

 and serves as a basis to parts of continents. We need 

 not be surprised, then, that the water helps the growth 

 and maintenance of these little quantities of living 

 matter of which the colony is composed. 



Besides the sponges and sea-anemones, the tank con- 

 tains other creatures, like them fixed to supports and 

 incapable of movement. Often these are completely 

 motionless, but occasionally, and suddenly, they quiver 

 and thus give an indication of their animal character. 

 There are the Lamellibranchiate or bivalve molluscs, 

 oysters, mussels and clams, which open and shut their 

 shells, and put out or draw in siphon-like tubes, which 

 they use for the intake and output of water. There are 

 also the Ascidians, sea-squirts, which were among the 

 precursors of the vertebrates. Attached to projecting 

 parts of the rocks, or sheltered in hollows, they pro- 

 trude their two gaping siphons. They continually 

 pump in water at one siphon and expel it at the other, 

 keeping a ceaseless stream passing through their 

 bodies. Sometimes they contract and draw in their 

 siphon-like tubes, but soon expand again, erecting 

 their siphons once more, thus indicating that this 

 circulation of water is a very important factor, and 

 performs a function they cannot do without. 



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