CLASSIFICATION OF ANIMALS 23 



11. Echinodermata (echinos, hedgehog; derma, skin). 

 Radially symmetrical animals, usually with five antimeres. 

 A large coelom is present, and there is a peculiar system 

 of tubes called the water-vascular system. An anus may 

 be lacking and is often non-functional when present. 

 There is usually a calcareous, spiny skeleton. Starfishes, 

 brittle-stars, sea-cucumbers, sea-urchins, stone-lilies. 4000 

 species. 



12. Annelida (annulus, ring). Metameric, bilaterally 

 symmetrical, triploblastic , soft-bodied worms. Body clearly 

 a tube within a tube i.e., coelom very well developed ; paired 

 excretory organs (nephridia) in each segment of the body; 

 no j ointed appendages. Earthworm, marine worms, leeches. 

 4000 species. 



13. Arthropoda (arthron, joint; pous, foot). Metameric, 

 bilaterally symmetrical animals; with a chitinous exo- 

 skeleton arid jointed appendages; coelom poorly developed. 

 Crustaceans, insects, spiders, centipedes, scorpions, ticks. 

 400,000 species. 



14. Mollusca (molis, soft). Non-met americ, bilaterally 

 symmetrical, triploblastic animals; in which there is a 

 well-developed digestive system with mouth and anus, a 

 small coelom, and usually a calcareous exoskeleton in the 

 form of a shell. Clams, mussels, snails, slugs, devil-fishes, 

 octopi. 60,000 species. 



15. Chordata (chorda, cord or string). Triploblastic, 

 metameric, bilaterally symmetrical animals; with well- 

 developed coelom and an endoskeleton the chorda or other 

 supporting structures. Paired appendages are usually 

 present, and these are often jointed. Fishes, newts, frogs, 

 reptiles, birds, mammals. 36,000 species. 



The relationships of the phyla are shown in Fig. 18. 



