24 COLLEGE ZOOLOGY 



(6) Nemathelminthes. — Thread Worms. Triploblastic ; bi- 

 laterally symmetrical; possess a tubular digestive system with 

 an anus; ccelom present. 1500. 



(7) Echinodermata. — Starfishes, Sea Cucumbers, Sea Ur- 

 chins, Sea Lilies. Triploblastic; radially symmetrical; usually 

 five antimeres; ccelom well developed; anus usually present; 

 locomotion in many species accomplished by characteristic organs 

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

 erally covers the body. 3000. 



(8) Annelida. — Jointed Worms. Triploblastic; bilaterally 

 symmetrical; ccelom well developed; anus present; segmented, 

 somites similar. 4000. 



(9) Mollusca. — Clams, Snails, Devilfishes. Triploblastic; 

 bilaterally symmetrical; anus and ccelom present; no segmenta- 

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

 muscular foot. 60,000. 



(10) Arthropoda. — Crabs, Insects, Spiders, Centipedes, 

 Scorpions, Ticks. Triploblastic ; bilaterally symmetrical ; 

 anus present; ccelom poorly developed; segmented, somites 

 usually more or less dissimilar ; paired, jointed appendages 

 present on all or a part of the somites; chitinous exoskeleton. 

 400,000. 



(11) Chordata. — Amphioxus, Sea Squirts, Vertebrates. 

 Triploblastic; bilaterally symmetrical; anus and ccelom present; 

 segmented; gill slits and a rod called the notochord present in 

 some stage of life history; central nervous system on dorsal side 

 of alimentary canal. 30,000. 



Zoologists do not agree as to the number of phyla into which 

 the animal kingdom should be divided. Some authorities recog- 

 nize only eight, while others maintain that there should be as 

 many as twenty, or even more. Two sub-kingdoms are generally 

 recognized, Protozoa (Phylum 1) and Metazoa (Phyla 2-1 1). 

 Recently many zoologists have come to believe that the sponges 

 (Phylum 2) should be separated from other Metazoa and called 

 the Parazoa. 



