156 ACORN-SHELL. 



when it is retracted into the body, and opens 

 again when protruded. It is hardly possible to 

 conceive a more wonderful structure in the whole 

 of the animal kingdom, and certainly not possible 

 to conceive one more beautiful, when the changing 

 tints of orange, scarlet, or azure are taken into 

 consideration. 



There is a kind of slimy muddiness about the 

 back of a sea-mouse that rather counteracts the 

 beautiful effect of its hairs. This is caused by 

 the muddy soil in which it loves to reside, and 

 which is strained through a dense mass of fine 

 hairs that interlace with each other, and arrest 

 the muddy particles, while they permit the water 

 for respiration to pass between them. The whale- 

 bone plates that fringe the mouth of the Green- 

 land whale have a somewhat similar office, only 

 the fringe of the whale catches molluscs, and 

 that of the aphrodite catches mud. 



Wherever rocks are found between tide-marks, 

 their surfaces are usually selected as resting- 

 places by some very curious animals, known by 

 the name of Acorn-shells, which will at once be 

 recognised by the sketch on plate n, fig. 3, where 

 is represented a group of these creatures which 

 have affixed themselves to the shell of a limpet. 

 In the original specimen, the entire surface of the 



