A GENERAL REVIEW OF THE ANIMAL KINGDOM 1 47 



is no notochord or vertebral column; the nervous- system is 

 chiefly ventral to the digestive tract; the heart, when present is 

 dorsal; and the blood usually has only colorless cells. The 

 principal phyla of the Invertebrates follow. 



181. Phylum Arthropoda {jointed feet). — This is by far the 

 largest phylum of the animal kingdom. It embraces crayfish, 

 lobsters, crabs (Crustacea), which for the most part have gills 

 and live in water; the Insects, as bees, flies, beetles, butterflies, 

 etc., which usually live in the air and get their oxygen from it; 

 the spiders and mites whose habits and appearance are somewhat 

 similar to those of the insects; and the myriapods, terrestrial 

 forms easily recognized by their elongated body composed of a 

 distinct head and a uniform segmented trunk bearing numerous 

 pairs of legs. Arthropods are especially to be recognized by 

 the fact that their bodies are segmented, are bilaterally sym- 

 metrical, and have paired jointed appendages to many of the 

 segments. In addition to this there is an external covering of 

 resistant substance (chitin). This serves for the protection of 

 the animal and for the attachment of the muscles within. 



182. Phylum Mollusca {soft). — This branch of the inverte- 

 brates includes the snail, clam and oyster, the squid and devil- 

 fish, and their kind. They differ very much among themselves 

 but agree in the lack of segmentation of their bodies, in the 

 absence of paired appendages, — and in those types most 

 commonly known to the student, in the presence of a shell 

 of one or two valves, which is secreted by a fold of the skin 

 called the mantle. 



183. Phyltan Echinodermata (spiny skin). — These are easily 

 recognized by^the possession of five or more arms or rays in 

 the adult sta^e. Usually a skeleton is developed in the skin. 

 This is often covered with spines, and from this fact the phylum 

 has its name. They are marine and are slow movers, — a few 

 being fixed by stalks to objects in the ocean. The starfish, 

 sea-urchin and sea-lilies are representatives. 



184. Phylum Annelida (Segmented Worms: with rings). — 

 This phylum is similar to the arthropods in that the body is 

 bilaterally symmetrical, is segmented, and sometimes has paired 



