APPENDIX B 629 



the Annelida. In the Arthropoda the first two characteristics are combined 

 with the presence of an exoskeleton and jointed appendages. In the 

 Chordata metamerism occurs in combination with a hollow dorsal nervous 

 system, notochord, and other peculiar features. 



7. Habitat. The environment in which an animal lives is not a criterion 

 for its classification; but information concerning the major habitat or 

 habitats of the members of a phylum aids in forming a conception of the 

 group. Thus all the Echinodermata are inhabitants of the ocean; the 

 greater number of the Platyhelminthes and at least half of the Nemathel- 

 minthes are parasitic; and very few of the Porifera occur in fresh water, 

 though all are aquatic. 



In the accompanying table of the chief groups of animals and in the 

 discussion of them that follows, extinct groups are starred. 



The Animal Phyla and Some of Their Subdivisions 

 SUBKINGDOM I. PROTOZOA 



The Unicellular Animals 



PHYLUM I. PROTOZOA. Animals composed of a single cell or of a colony of 

 similar cells. 



SUBKINGDOM II. METAZOA 



Multicellular Animals with Specialized Tissues 



(1) Metazoa without a true digestive cavity 

 PHYLUM II. PORIFERA. The Sponges. Diploblastic, phagocytic. 



(2) Metazoa with a true digestive cavity with 



mouth but no anus; coelom absent 

 PHYLUM III. COELENTERATA. The Coelenterates : Hydra, corals, jelly- 

 fishes, sea anemones, etc. Dipoblastic; with stinging cells. 

 PHYLUM IV. CTENOPHORA. The Comb Jellies or Sea Walnuts. Transitional 

 between diplo- and triploblastic ; without stinging cells. 



(All remaining phyla triploblastic) 

 PHYLUM V. PLATYHELMINTHES. The Flatworms: Planaria, liver flukes, 

 tapeworms, etc. 



(3) Metazoa with coelom in addition to digestive 

 cavity; mouth and anus present; body built 



on tube-within-tube plan 

 (Body unsegmented = non-metameric) 

 PHYLUM VI. NEMATHELMINTHES. The Roundworms: Nematodes, etc. 

 PHYLUM VII. ECHINODERMATA. The Echinoderms. Radially symmetrical 



or with secondary bilateral symmetry. 



Class 1. Starfishes. (Asteroidea.) 



Class 2. Serpent Stars or Brittle Stars. (Ophiuroidea.) 



Class 3. Sea Urchins and Sand Dollars. (Echinoidea.) 



Class 4. Sea Cucumbers. (Holothuroidea.) 



Class 5. Sea Lilies or Feather Stars. (Crinoidea.) 



