138 ZOOLOGY. 



blood has red corpuscles. They are called Chordata or Verte- 

 brata. All other animals, with the exception of a few which 

 seem intermediate in some respects, are classed as Inverte- 

 brates, and agree in general in the following 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. 



176. Arthropoda. 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 sym- 

 metrical, 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. 



177. Mollusca. This phylum of invertebrates 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 ap- 

 pendages, 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 intelli- 

 gence, one group of them that which includes the squid, 

 has the most highly developed brain found below the verte- 

 brates. It occupies among the invertebrates somewhat the 

 place which man has among the mammals. 



178. Echinoderms. These are easily recognized by the 

 possession of five or more arms or rays in the adult stage. 



