304 CHARACTERS OF INVERTEBRATE ANIMALS 



though not the cord. In no Invertebrate does the central nervous 

 system consist, as in Vertebrates, of a hollow cord running along 

 the dorsal side of the body. 



The contrasts between a Vertebrate and a higher Invertebrate 

 are illustrated by the accompanying diagrams (fig. 176). 



The following groups or phyla of the Invertebrata are recog- 

 nized. It must not be imagined, however, that they are anything 

 like of equal size, for some are exceedingly large, while others are 

 relatively small. 



I. Nemertines (Nemertea). — ^Worm-like marine forms, which in some 

 respects approach the Chordata in structure. 



II. Molluscs (Mollusca), including such forms as Cuttle- Fishes, Snails, 

 Slugs, Oysters, and Mussels. 



III. Jointed-limbed Animals (Arthropoda), the largest group of the 

 animal kingdom, comprising such creatures as Insects, Scorpions, Spiders, 

 and Mites; Centipedes and Millipedes; Shrimps, Lobsters, and Crabs. 



IV. Segmented Worms (ANNELIDA). — A large group of forms, in- 

 cluding innumerable Marine Worms (free-living and tube- inhabiting). 

 Earth- Worms, Fresh-water Worms, and Leeches. 



V. Siphon- Worms (Gephyrea). 



VI. Wheel Animalcules (Rotifera). 



VII. Moss-Polypes and Lamp-Shells (Molluscoida). — The great 

 bulk of these are fixed marine animals, and those belonging to the first 

 group are nearly all colonial. 



VIII. Flat-Worms (Platyhelmia). — The most familiar of the forms 

 grouped here are the Flukes and Tape- Worms, which are found as parasites 

 within the bodies of other animals. 



IX. Thread- Worms (Nemathelmia). — The name of the group indi- 

 cates the shape of these creatures, most of which are parasites, either in 

 plants or else within the bodies of other animals. 



X. Echinoderms (Echinodermata). — This phylum is constituted by 

 such marine forms as Star-Fishes, Sea-Urchins, Sea- Lilies, and Sea- 

 Cucumbers. 



XI. Zoophytes (Ccelenterata). — Mostly marine animals, which may 

 be either simple or colonial, fixed or free-swimming. Familiar examples 

 are Jelly-Fishes, Sea-Anemones, and Corals. 



XII. Sponges (Porifera). — Mostly marine, colonial, and fixed. 



XIII. Animalcules (Protozoa). — This lowest phylum includes an 

 immense number of simply-constructed animals, which are nearly always 

 very small or microscopic in size. They are found almost everywhere, but 

 are unfamiliar to those who are not in the habit of using the compound 

 microscope. 



