90 A HALF-HOUR WITH THE 



outer skeleton of the lower animals, as tbe mol- 

 luscs, crabs, and fishes ; the internal skeleton of 

 the higher animals affords a not less interesting 

 field of research. If we take a piece of bone, and 

 having ground it so fine that we may examine it 

 with transmitted light under the Microscope, we 

 shall find it composed of a number of minute 

 insect-shaped cells, surrounding an open canal, as 

 seen at figure 232, plate 8. These cells, which are 

 called lacuncB, and their little branches canaliculi, 

 are modifications of the cells found in fishes' scales, 

 and figured at 175, plate 6. 



These curiously-shaped cells differ in size and 

 form in the various classes of animals belonging to 

 the sub-kingdom Vertebrata, and thus a small por- 

 tion of a bone will frequently serve to indicate 

 whether an animal belonged to fishes, reptiles, birds, 

 or mammals. This is a matter of importance to 

 the geologist in determining the character of the 

 inhabitants of the earth at former periods of its 

 history. A section of whalebone is figured at 242, 

 plate 8. 



The shells of eggs seems to be formed on the 

 same general principles as other hard parts, and the 

 tendency to the formation of cells with canaliculi 

 may be easily observed, as in the section of a com- 

 mon egg-shell, represented at figure 181, plate 6. 

 The young egg-shell should be examined, a section 

 of which is seen at 182, if the object is to study the 

 history of the development of the shell ; and this 

 may be compared on the one hand with the shells 

 of the Mollusca and the Crustacea, and on the other 

 hand with those of the scales, teeth, and bones of 

 the vertebrate animals. Egg-shells present very 

 different appearances. The shell of the emu, for 

 instance, exhibits a series of dark triangular spota, 

 and is represented at figure 183, plate 6. 



