THE CLASSIFICATION OF ANIMALS 109 



example, the present-day 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 relation- 

 ships among animals is by studying their development. 

 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 correspon- 

 dence, 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 different 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 example, certain animals belonging 

 to the group which includes the crabs, lobsters, and cray- 

 fishes, have adopted a parasitic 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 re- 

 remblances and differences, we should never classify the 

 Sacculina (the parasite) in the group Crustacea, which is 

 the group including 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 study 

 of the development of Sacculina we find that it is more 



