6 AN INTRODUCTION TO ZOOLOGY 



metry usually distorted by colonial habits; ciliated tentacles 

 about the mouth; coelom and anus present. 700 (1800). 



9. Brachiopoda. Tongue or lamp shells. Triploblastic; 

 bilaterally symmetrical; ccelom present; anus present or absent; 

 ciliated arms around the mouth; body covered by a shell com- 

 posed of dorsal and ventral valves. 100 (2500). 



10. Echinodermata. Starfishes, sea cucumbers, sea urchins, 

 sea lilies. Triploblastic; radially symmetrical, usually five 

 antimeres; coelom well developed; anus usually present; loco- 

 motion in many species accomplished by characteristic organs 

 known as tube feet; a spiny skeleton of calcareous plates gener- 

 ally covers the body. 2500 (2600). 



11. Annelida. Jointed worms. Triploblastic; bilaterally 

 symmetrical; coelom well developed; anus present; segmented, 

 somites similar. 2500 (25). 



12. Arthropoda. Crabs, insects, spiders, centipedes, scor- 

 pions, ticks. Triploblastic; bilaterally symmetrical; anus 

 present; coelom poorly developed; segmented, somites usually 

 more or less dissimilar; paired jointed appendages present on all 

 or a part of the somites; chitinous exoskeleton. 200,000 (5000). 



13. Mollusca. Clams, snails, devilfishes. Triploblastic; 

 bilaterally symmetrical; anus and coelom present; no segmenta- 

 tion; shell usually present; the characteristic organ is a ventral 

 muscular foot. 22,000 (21,000). 



14. Tunicate. Sea squirts. Triploblastic; bilaterally sym- 

 metrical; coelom and anus present; form saccular or barrel- 

 shaped; two apertures through which water currents pass in and 

 out; frequently colonial; all pass through a tadpole-like larval 

 stage during which the body is supported by a rod-like structure, 

 the chorda. 300 (no fossils). 



15. Vertebrata. Bilaterally symmetrical; coelom well de- 

 veloped; anus present; chorda usually more or less fully replaced 

 by a series of bony vertebrae; many systems of internal organs 

 segmented; two pairs of appendages usually present; central 

 nervous system always dorsal to digestive tube. 25,000 (2400). 



