RUDIMENTARY ORGANS OF ANIMALS 



131 



Haliotes, and Fissurella are bilaterally symmetri- 

 cal, while in other Gastropods the spiral twisting 

 of the body causes a progressive diminution of the 

 organs situated on the side towards which the 

 twisting occurs ; the organs of the left side may 

 therefore become smaller, and finally atrophy 

 almost completely. 



The internal shell of the Cephalopods furnishes 

 a striking example of 

 a rudimentary organ. 1 

 The Nautilus (fig. 54) 

 has a shell, the spiral 

 coils of which are 

 pressed tightly against 

 one another. The spiral 

 is divided into a series 

 of chambers by means 

 of partitions, each par- 

 tition being provided 

 with an aperture for 

 the admission of the 

 siphon (7.). The shell 

 of Spirula (fig. 55 and 56, A), a creature still existing, 

 is only partially curled round ; the last chamber of 

 the shell is very small, and only encloses a part of 

 the animal, most of which remains outside the shell, 

 and partially covers it by the mantle (p.), the 

 shell being therefore partly external and partly 

 internal. On examining the fossil species : Spiru- 



1 W. Boas, Lehrbuch der Zoologie. Jena, 1894. 



a 



FIG. 54. Nautilus pompilius. 

 terminal chamber of the shell ; a, body 

 of the animal ; /, siphon ; r, mantle fold. 

 (After Owen.) 



