4 COLLEGE ZOOLOGY 



the presence of eight legs. Many of them, like the scorpion, 

 are capable of inflicting severe wounds with their stings. The 

 curious king-crab is now placed by zoologists in the group ARACH- 

 NIDA. 



The INSECTA are the butterflies, bees, beetles, bugs, etc. They 

 have six legs, and usually possess wings. 



The MYRIAPODA are long, slender, terrestrial animals with 

 one or two pairs of legs on each body segment; they are known as 

 centipedes (Fig. 233) and millipedes (Fig. 232). 



The CRUSTACEA are mainly aquatic ARTHROPODA, and breathe 

 with gills; they include the lobsters, crayfishes, crabs, barnacles, 

 sow bugs, and many others. 



(3) The Mollusca. - - The MOLLUSCA (Chap. XII) most often 

 seen are the snails and clams; the slug, oyster, squid (Fig. 191). 

 nautilus (Fig. 194), cuttlefish, and octopus, are also well known. 

 They are of various shapes and sizes, but most of them possess a 

 ventral muscular structure called the foot, which usually serves 

 as an organ of locomotion. Often a heavy shell of calcium car- 

 bonate covers the body. 



(4) The Annelida. The ANNELIDA (Chap. XI) are known 

 as segmented worms, since their bodies consist of sometimes over 

 one hundred rings or segments and their shape is wormlike. 

 The earthworm is the commonest representative of the group. 

 There are many marine annelids, and also a number of fresh- 

 water members, like the leech. The medicinal leech (Fig. 169) 

 is famous for its use in sucking blood. 



(5) The Echinodermata. - - The starfish (Fig. 131) is a well- 

 known echinoderm, and usually serves as a type of the group. 

 Like all echinoderms, it is radially symmetrical, and has five arms 

 extending out from a central disc. The other echinoderms are 

 called brittle stars, sea urchins, sea cucumbers, and sea lilies. 

 Most of these animals have a body-wall supplied with spicules 

 of calcium carbonate; hence their name, which means spiny- 

 skinned. They all live in salt water, and are therefore seldom 

 seen by people who do not visit the sea coast. 



