CLASSIFICATION 527 



sponge, the waste products being discharged through larger exhalant 

 openings. The Porifera have no body cavity, but often secrete a 

 horny (bath sponge), calcareous, or siliceous skeleton, 



Phylum III. Ccelentera. Multicellular animals which, like the 

 Porifera, have no body cavity ; they are usually radially sym- 

 metrical and develop stinging cells. The most familiar examples 

 are the sea-anemones, jelly-fishes, and corals, the last-named being 

 colonies of individuals which secrete a calcareous skeleton. 



Phylum IV. Platyhelminthes or Flat-worms. In this group the 

 primitive radial symmetry is lost ; the body is flattened but no true 

 body cavity is formed. Here the first indications of a head and brain 

 are to be discerned. To this phylum belong the Trematodes or 

 Flukes (liver fluke of the sheep) and the Cestodes or Tapeworms, 

 all of which are parasitic. 



Phylum V.Nemertea or RMon Worms. These are the first 

 animals possessing an open gut and closed blood-system ; they are 

 mostly marine and, for the purpose in hand, unimportant. 



Phylum VI. Nematohelminthes. Thread-like worms, mostly para- 

 sitic and provided with a distinct body cavity, a well-developed 

 alimentary canal, a mouth, and an anus. To this phylum belong 

 such well-known parasites as the round- worm (Ascaris), the thread- 

 worm (Oxyuris), the Guinea worm (Dracunculus) , Trichina, &c. 



Phylum VII. Annelida. These exhibit a well- developed ccelom 

 and a distinct segmentation of body which is visible externally. The 

 Annelida include the Chaetopoda or worms with bristles (earth-worm, 

 lob- worm) and the Hirudinea or worms devoid of bristles (leech). 



Phylum VIII. Echinoderma. The echinoderms are radially 

 symmetrical animals which exhibit a tendency to form calcareous 

 skeletons. Familiar examples may be found in the sea-cucumbers, 

 sea-urchins, star-fishes, &c. 



Phylum IX. Arthropoda. This important phylum consists of 

 bilaterally symmetrical animals exhibiting numerous segments and 

 bearing paired, jointed appendages ; the sexes are always separate. 

 The chief classes are Crustacea, Myriopoda, Insecta, and Arachnoidea. 



The Crustaceans include the crabs, lobsters, cray-fish, shrimps, &c. ; 

 they mostly live in water and breathe by gills. 



The Myriopods embrace the centipedes, millipedes, &c. 



The Insects are usually winged, breathe by a system of air-tubes 

 and undergo a metamorphosis. They are subdivided into eighteen 

 orders of which the following are the most important for the purposes 

 of the present work. 



(a) Hymenoptera. Ants, bees, wasps, &c. ; they have four 

 transparent wings, are usually provided with mandibles, 

 and are furnished with a sting or with an ovipositor. 



