capturing prey; with a simple blood vascular 

 system. (500 species.) 



Phylum NEMATHELMINTHES: the round- 

 worms; a fairly large phylum; characterized 

 by elongated, cylindrical, bilaterally sym- 

 metrical bodies; live as parasites in plants 

 and animals, or are free-living in the soil or 

 water. (12,000 species.) 



Phylum ACANTHOCEPHALA: the hook- 

 headed worms; parasitic worms with no di- 

 gestive tract and a head armed with many 

 recurved hooks. (100 species.) 



Phylum CHAETOGNATHA: free-swimming 

 marine worms, with a body cavity (coelom) 

 that develops from pouches of the digestive 

 tract (as in the echinoderms and lower chor- 

 dates). (30 species.) 



Phylum NEMATOMORPHA: the horsehair 

 worms; extremely thin, brown or black worms 

 about 6 inches long, resembling a horsehair; 

 adults are free-living, but the larvae are para- 

 sitic in insects. (200 species.) 



Phylum ROTIFERA: small, wormlike ani- 

 mals, commonly called "wheel animalcules," 

 with a complete digestive tract, flame cells, 

 and a circle of cilia on the head, the beating 

 of which suggests a wheel. (1200 species.) 



Phylum GASTROTRICHA: microscopic, 

 wormlike animals resembling the rotifers, 

 but lacking the crownlike circle of cilia. (100 

 species.) 



Phylum BRYOZOA: "moss" animals; micro- 

 scopic organisms, usually marine, which form 

 branching colonies; characterized by a U- 

 shaped row of ciliated tentacles, the lopho- 

 phore, by means of which they capture food. 

 (1200 species.) 



Phylum BRACHIOPODA: the lamp shells; 

 marine animals with two hard shells (one 

 dorsal and one ventral), superficially like a 

 clam; obtain food by means of a lophophore. 

 (200 species at present; 3000 extinct.) 



Phylum PHORONIDEA: wormlike, marine 

 forms that secrete and live in a leathery tube; 

 they have a U-shaped digestive tract and a 

 lophophore. (10 species.) 



Phylum ANNELIDA: segmented worms, usu- 

 ally with well-developed coelom, head, and 



Classification of Organisms - 683 



blood vessels; appendages, when present, are 

 nonjointed. (7000 species.) 



Class Archiannelida: marine worms, sim- 

 pler than other annelids. 



Class Polychaeta: mainly marine worms, 

 with a distinct head and many-bristled lobose 

 appendages. 



Class Oligochaeta: mainly fresh-water an- 

 nelids, with scanty bristles and usually a 

 poorly differentiated head. 



Class Hirudinea: leeches. 



Phylum MOLLUSCA: unsegmented animals, 

 usually with one or more shells; a foot, vari- 

 ously modified, and a mantle are character- 

 istic. (75,000 species.) 



Class Amphineura: the chitons, etc.; sim- 

 plest type of mollusks, with shell in eight 

 pieces. 



Class Pelecypoda: bivalve mollusks; clams, 

 oysters, mussels, scallops, etc.; usually with a 

 hatchet-shaped foot and no distinct head. 



Class Scaphopoda: the tooth shells; mol- 

 lusks with a conical tubular shell. 



Class Gastropoda: asymmetrical mollusks, 

 usually with a spiral shell; snails, etc. 



Class Cephalopoda: octopus, squid, nau- 

 tilus, etc.; "headfoot" with 8 or 10 tentacles; 

 well-developed eyes and nervous system. 



Phylum ARTHROPODA: segmented ani- 

 mals, jointed appendages, no endoskeleton. 

 (700,000 species.) 



Class Crustacea: lobsters, crabs, crayfishes, 

 shrimps, etc. 



Class Arachnida: spiders, mites, scorpions, 

 etc. 



Class Onychophora: simple, wormlike, ter- 

 restrial arthropods; Peripatus. 



Class Insecta: insects. Example: Drosoph- 

 ila. 



Class Chilopoda: the centipedes; at least 

 15 segments, each with one pair of jointed 

 appendages. 



Class Diplopoda: the millipedes; abdomen 

 with many segments, each with two pairs of 

 appendages. 



Phylum ECHINODERMATA: radially sym- 

 metrical in adult stage; with well-developed 

 coelom formed from enteric pouches; with 



