INTRODUCTION 



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Phylum Arthropoda. — The representatives of this group may be 

 aquatic or terrestrial. Their bodies are segmented, and they have 

 segmented appendages. The group includes crayfishes, lobsters, 

 crabs, centipedes, scorpions, and all insects, such as bugs, beetles, 

 butterflies, flies, etc. This is by far the largest single phylum. Some 

 authors believe as many as 675,000 species belong to it. 



Phylum Mollusca. — Unsegmeuted animals that are usually en- 

 closed in a calcareous shell. The single muscular "foot" is a char- 

 acteristic structure. Common forms include clams, snails, slugs, 

 and octopuses. About 78,000 species have been recognized. 



Phylum Chordata. — Segmentally constructed animals with bilat- 

 eral symmetry and an endoskeletal axis or notochord at some stage. 

 Many of our best known animals belong here ; the phylum includes 

 lampreys, sharks, bony fish, frogs, salamanders, alligators, snakes, 

 turtles, rats, birds, horses, sheep, cows, monkeys, and men. Ap- 

 proximately 40,000 species have been described in the group. 



In addition to the above generally recognized phyla, there are 

 several other more or less independent smaller but distinct groups. 

 Most of these groups have certain of the wormlike characteristics. 

 Many authors have dignified each of these as a phylum. They are : 

 Nemertinea, — nearly unsegmented, contractile, wormlike forms; 

 Trochelminthes — unsegmented and frequently similar to certain 

 larval stages of annelids and molluscs, rotifers being typical; 

 Bryozoa — colonial, marine, or fresh-water forms, of which there are 

 about 1,750 known species ; Brachiopoda — marine animals enclosed 

 in a bivalve shell, the majority of which are fossil; Phoronidea — 

 sessile marine worms living in chitinous tubes in shallow water; 

 Chaetognatha — marine, transparent, carnivorous worms of which 

 Sagitta is an example; Sipunculoidea — unsegmented, elongated 

 marine worms, living either free, in tubes, or in snail shells. A 

 number of these are sometimes described under the phylum name 

 MoUuscoida. 



Vital Relations of Animals and Plants 



There are certain single-celled organisms that are claimed as 

 animals by zoologists and as plants by botanists. As a matter of 

 fact, it is not easy to draw an absolutely clear-cut line of distinc- 

 tion. Of course, it is easy to recognize the extremes. Anyone 



