CLASSIFICATION OF ANIMALS. 21 



(i) crayfish, lobsters, crabs (Crustacea), which for the most part 

 have gills and live in water; (2) the Insects as bees, flies, beetles, 

 butterflies, etc., which usually live in the air and get their oxygen 

 from it; (3) the Spiders, whose habits and appearance are somewhat 

 similar to those of the insects. The arthropods are to be recognized 

 by the fact that their bodies are made up of segments, are bilaterally 

 symmetrical (that is, have a right and a left side), and have paired 

 jointed appendages to many of the segments. In addition to this, 

 there is an outer protective covering (skeleton) developed by the 

 skin. This is resistant and serves for the attachment of the muscles 

 which lie within it. 



27. Mollusks. This phylum of the invertebrates includes the 

 snail, the clam and oyster, the squid and devil-fish, and their kind. 

 They differ very much among themselves, but they agree in the lack 

 of segmentation of the body, in the absence of paired appendages, 

 and in those types most commonly known to the student in the 

 presence of a protective limy shell, of one or two "valves," which 

 is secreted by a fold of the skin known as the mantle. Most of the 

 mollusks are lowly in organization and in intelligence; the squid, 

 however, has the most highly developed brain found below the 

 vertebrates. It occupies among the invertebrates somewhat the 

 place which man has among the vertebrates. 



28. Echinoderms. These are easily recognized in the adult stage 

 by the five or more arms or rays that radiate from the central point. 

 Because of this arrangement they are described as radially sym- 

 metrical. Usually a skeleton, bearing spines, is developed in the 

 skin. They are found only in the sea and are poor movers some of 

 them being actually attached to the bottom by stalks. The star- 

 fish, sea-urchin, and sea-lilies are representatives. 



29. Annulata (Segmented Worms). The earthworm is the best 

 known representative of this group. Annulata are similar to the 

 arthropods in several particulars. They are bilaterally symmetrical, 

 the body is segmented and has paired appendages on many of the 

 segments. The appendages, however, are not jointed, but are 

 merely setx or hairs, in sockets or on fleshy prominences. The 

 segments are more nearly alike in the different parts of the body 

 than they are in the typical arthropods. The larval stage of insects 

 (caterpillars, grubs, maggots, etc.) are likely to be confused with 

 the segmented worms, if the student is not watchful. Besides the 

 earthworm, the leeches and many aquatic worms, both fresh water 

 and marine, are included here. 



