24 ESSENTIALS OF ZOOLOGY 



Phylum Annelida (Jointed worms). — This group is characterized 

 by segmented body, well-developed body cavity, and nephridia as 

 tubular excretory structures. They live in marine waters, fresh 

 water, and in the soil. The earthworm and leech are well-known 

 examples of the phylum. There are at least 4,500 known species. 



Phylum Arthropoda. — The group includes crayfishes, lobsters, 

 crabs, centipedes, scorpions, and all insects. Their bodies are seg- 

 mented, and they have segmented appendages. This is by far the 

 largest single phylum. Some authors believe as many as 675,000 

 species belong to it. 



Phylum MoUusca. — Unsegmented 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. Approx- 

 imately 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 ; Tro- 

 chelminthes — 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 ; Chaetog- 

 natha — 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 Molluscoida. 



Balance in Nature 



The influence exerted by one animal or one group of animals on 

 another can hardly be estimated until one of them leaves the pic- 

 ture. In an established animal community which might be said to 



