INSECT'S STOMACH SNODGEASS 369 



First, it must be understood that there is a pretty sharp division 

 in the animal kingdom between the one-celled animals and the many- 

 celled animals. The first are the Protozoa, the second the Metazoa. 

 The Metazoa are the true stomach animals; they include everything 

 from the sponges to us. Being confronted at the very outset of their 

 career with the necessity of feeding a mass of cells that had lost 

 their individualities, the Metazoa proceeded at once to the organi- 

 zation of a community dietary department. 



There are three gi'oups of metazoic animals that we shall mention 

 particularly in connection with a study of the evolution of the 

 insect stomach. The first group is the Coelenterata, which includes 

 the hydras, the sea anemones, the coral polyps, and the jellyfish. 

 These animals are principally double-walled stomachs. They seem 

 to live very well without complicating themselves with other organs 

 of any consequence; they have only the simplest kind of nervous 

 system and a contractile tissue that passes for muscle, but they have 

 no legs or other external appendages except a circle of soft tentacles 

 around the mouth. The second group to be mentioned is the seg- 

 mented worms, or Annelida, including the earthworms and many 

 kinds of worms that live in the ocean. The ancestors of the worms 

 apparently were too progressive to put up with the simple equipment 

 of a coelenterate. They started new ways of doing things and 

 equipped themselves inside and out with new apparatus that com- 

 plicated their anatomy and raised their rating in the scheme of 

 zoological classification, but it is not clear that they gained any 

 material advantage over the coelenterates or added to their own 

 happiness in any way. A curious land animal named Peripatus 

 that looks like a worm with a long series of caterpillar legs is 

 probably a relative of the annelids. The third principal group is 

 the Arthropoda, the members of which we know more familiarly as 

 the spiders, centipedes, shrimps, crayfish, lobsters, crabs, and insects. 

 The arthropods ad()])ted most of the ideas and organs of the anne- 

 lids, but added to the worm equipment a double scries of jointed 

 legs. The arthropods have become so efficient in a mechanical way 

 that some scientists think there is danger of their being eventually 

 masters of the earth. Evidently they have at least found a modus 

 of living with their own machinery, while ours, it is said, threatens 

 to displace us. The secret of the arthropods is that each individual 

 is his own machine. 



Lest the reader begin to feel that this digression about jellyfish, 

 worms, and arthropods has nothing to do with the subject of 

 stomachs, it must be explained at once that all the important struc- 

 tural features of animals have been evolved around the stomach; 

 that is, they have been developed as adaptations to feeding, or its 



