302 COLLEGE ZOOLOGY 



living among seaweeds; it is almost transparent. The hermit- 

 crab, Eupagurm, lives in an empty snail-shell which protects it 

 from many enemies. Some hermit-crabs place sea-anemones 

 or hydroid colonies upon their shells; these furnish additional 

 protection. 



The edible or blue crab, Callinectes, lives along the Atlantic 

 and Gulf coasts and is captured in large numbers for market. 

 It is called the soft-shelled crab just after molting. The fid- 

 dler-crabs, Uca pugilator, are common along our eastern coast, 

 where they dig holes in the mud and sand. The spider-crab, 

 Libinia, has long slender legs, which enable it to run over uneven 

 surfaces with ease. The Japanese spider-crab is very large, 

 sometimes measuring twenty feet across from tip to tip of the 

 first pair of legs. 



(5) The Biogenetic Law. - - Early in the past century it was 

 noticed that animals could be arranged in a series beginning with 

 the PROTOZOA and passing through the simpler diploblastic forms, 

 and that the stages in this series correspond to the early stages 

 in the embryology of the METAZOA. This led to the formulation 

 of the biogenetic law, i.e. that the development of the individual 

 recapitulates the stages in the evolution of the race, or ontogenv 

 recapitulates phylogeny. These stages contrasted appear as 

 follows : - 



PHYLOGENETIC STAGE ONTOGENETIC STAGE 



(1) Single-celled, animal Egg cell 



(2) Ball of cells Blastula 



(3) Two-layered sac Gastrula 



(4) Triploblastic animal Three-layered embryo 



Zoologists soon became interested in the recapitulation theory, 

 and enlarged upon it. Of these, Fritz Miiller and Ernst Haeckel 

 are especially worthy of mention. The latter expressed the facts 

 as he saw them in his " fundamental law of biogenesis." The 

 ancestor of the many-celled animals was conceived by him as 



