MECHANICS OF RESPIRATION 



so that there is only one way out for the water, through 

 the siphon. 



The poulps or octopuses, behaving in the same way, 

 exhibit the same method but on a larger scale. They 

 have not, like the cuttle-fish, a structure of hard 

 skeletal substance in their bodies, nor have they one 

 around them ; there is nothing to limit extension or con- 

 traction, so they are able to swell out, as well as to 

 empty themselves, to an extreme degree. Their elastic 

 bodies swell out and collapse like india-rubber bladders. 

 According to circumstances, these bodies change their 

 form and size, sometimes blown-out and almost 

 spherical, sometimes shrivelled and collapsed, so as 

 to find their way into the hollow of a rock. 



Other aquatic creatures also have a mechanism of 

 respiration, but one that is less varied and more 

 tranquil; their functional attitudes remain the same 

 for a longer time ; the renewing of the water goes on 

 continuously and is not divided, or is less divided, 

 into two opposed phenomena, inspiration and ex- 

 piration. The clam and the bivalve molluscs which 

 resemble them put out from their shells the two 

 tubes of their siphons, one serving to take the water 

 to the gills in a sheltered position inside, the other to 

 send it out again. Other bivalves, like oysters and 

 mussels, which have no siphons, half-open their shells, 

 and make apertures through which the water can come 

 in and out, so that it is constantly renewed. 



In other cases, respiration is managed more simply 

 still. In the absence of inner respiratory organs to 

 which it would be necessary to guide the water, the 

 movements are limited to extruding at intervals the 

 parts which act as gills, then withdrawing them again. 

 The tubicolous worms, so called because they make a 

 shelter for themselves in which to live, behave in this 

 way. The Protules, in their whitish, sinuous tubes, 

 send out, if there is nothing to disturb them, the red 

 plume of filamentous gills which adorns their heads; 

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