A GENERAL REVIEW OF THE ANIMAL KINGDOM 145 



ing facts: there 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 the most 

 numerous 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, whose habits and appearance are 

 somewhat similar to those of the insects. Arthropods are 

 especially to be recognized by the fact that their bodies are 

 segmented, are bilaterally symmetrical, and have paired jointed 

 appendages to many of the segments. In addition to this there 

 is a covering of resistant substance (chitin) developed by the 

 skin. 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. While many of the mollusks are lowly in 

 organization and in intelligence, one group of them that which 

 includes the squid, has the most highly developed brain 

 found below the vertebrates. It occupies among the inverte-, 

 brates somewhat the place which man has among the mammals. 



183. Phylum Echinodermata (spiny skin). These are easily 

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

 the adult stage'. 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 poor movers, a few 

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

 sea-urchin and sea-lilies are representatives. 



