MOLLUSCS WHICH BREATHE IN WATER 393 



Shell's gills. It is in fact a case where efficiency has been gained 

 by the suppression of some of the members of a series (see p. 384), 

 those remaining becoming larger and more specialized. In an 

 active rapacious animal like a Cuttle-Fish some special method of 

 constantly renewing the water in the large gill-cavity is an obvious 

 necessity, especially as all the waste products of the body (in addi- 

 tion to carbonic acid gas from breathing) are discharged into it. 

 We find that the back wall of the cavity is extremely muscular, 

 and during life contracts at regular intervals, as can readily be seen 

 in an aquarium specimen, so as to force out the impure water 

 and the various waste substances. These are expelled through 

 a muscular tube, the funnel, pure water being admitted by the 

 large slit already mentioned. As will be explained in another 

 section, the waste water is ejected with such force through the 

 funnel that it enables the animal to swim backward with great 

 velocity. As in so many other cases, various organs of the body 

 are always liable to have fresh duties imposed upon them, in 

 addition to their own proper and original work. And as in the 

 course of time such an organ often gives up the old work entirely, 

 and devotes itself wholly to the new, we find that the animal 

 kingdom presents numerous cases of "changes of function". 



The breathing apparatus of the Pearly Nautilus {Nautilus 

 pompilius] is a good deal like that of the Cuttle-Fish, but there 

 are four gills instead of two, and the funnel is of simpler nature, 

 consisting of two halves rolled together but not fused. 



SNAILS AND SLUGS (GASTROPODA) 



The typical SEA-SNAILS and SLUGS (marine Gastropods) agree 

 to some extent with Cuttle- Fishes, for they have given up the 

 series of gills which their remote ancestors probably possessed, 

 and those which remain are sheltered in a comparatively spacious 

 gill-cavity, into which all the waste products of the body are 

 discharged. But as a consequence of an extraordinary twisting 

 round of the upper parts of the body which has taken place, one 

 result of which is the spiral shape of the shell, this cavity is placed 

 in front, being so to speak over the shoulders, and opening by 

 a large slit above the neck region. In some cases two gills are 

 present, e.g. in the Ormer or Sea-Ear (Haliotis] (fig. 523), and 

 the waste water makes its exit by a slit in the roof of the gill- 



