THE CLASSIFICATION OF ANIMALS 67 



one-toed horse has been clearly shown by series of fossils 

 to be descended from a small five-toed horse-like animal 

 which lived in the Tertiary age. 



Importance of development in determining classifica- 

 tion. A very important means of determining the 

 relationships among animals is by studying their develop- 

 ment. If two kinds of animals undergo very similar 

 development, that is, if in their development and growth 

 from egg-cell to adult they pass through similar stages, 

 they are nearly related. And by the correspondence or 

 lack of correspondence, by the similarity or dissimilarity 

 of the course of development of different animals much 

 regarding their relationship to each other is revealed. 

 Sometimes two kinds of animals which are really nearly 

 related come to differ very much in appearance in their 

 fully developed adult condition because of the widely differ- 

 ent life-habits the two may have. But if they are nearly 

 related their developmental stages will be closely similar 

 until the animals are almost fully developed. For exam- 

 ple, certain animals belonging to the group which includes 

 the crabs, lobsters, and crayfishes, have adopted a para- 

 sitic habit of life, and in their adult condition live attached 

 to the bodies of certain kinds of true crabs. As these 

 parasites have no need of moving about, being carried by 

 their hosts, they have lost their legs by degeneration, and 

 the body has come to be a mere sac-like pulsating mass, 

 attached to the host by slender root-like processes, and 

 not resembling at all the bodies of their relatives the 

 crabs and crayfishes. If we had to trust, in making out 

 our classification, solely to structural resemblances and 

 differences, we should never classify the Sacculina (the 

 parasite) in the group Crustacea, which is the group in- 

 cluding the crabs and lobsters and crayfishes. But the 

 young Sacculina is an active free-swimming creature 

 resembling the young crabs and young shrimps. By a 



