A HISTOEY 



OF 



EECENT CEUSTACEA 



THE MALACOSTBAGA 



CHAPTER I 



OUTLINE OF CLASSIFICATION 



It is conceivable that by origin all the animals of the globe 

 belong to a single family. They now exhibit very great 

 divergence. Between a star-fish and a crocodile, for ex- 

 ample, the cousin ship is obscure and remote. Yet almost 

 all species may be included within a few principal clans, 

 and these are united one to another by a small number of 

 intermediate forms of life. For the whole series the de- 

 tails of classification will vary with the increase of know- 

 ledge. No system has yet been accepted as final. One, 

 which is sufficiently good for our present purpose, dis- 

 tributes animals among nine leading divisions. These are 

 (1) the Protozoa, primitive animals, such as the Foramini- 

 fera and Infusoria ; (2) the Coelenterata, in which the body- 

 cavity serves alike for circulation and digestion, a tribe 

 which includes sponges, corals, and jelly-fish ; (3) the 

 Echinodermata or prickly- skinned animals, embracing the 

 sea-lilies, star-fishes, sea-urchins, sea-cucumbers, and a 



