112 ELEMENTS OF BIOLOGY 



Class in. GASTROPODA (Fig. 54). Asymmetrical forms with rather 

 large and well-developed heads. The shell when present is spiral. 

 Snails and Slugs are common examples. 



Class IV. scAPHOPODA (Fig. 55). The mantle forms a tube which 

 secretes a tubular shell open at both ends and larger at one end 

 than at the other. The Tooth Shell, Dentalium, is a type form. 



Class V. CEPHALOPODA (Fig. 56). Outwardly to the uninformed 

 observer the Squids, Octopods, and similar forms which make up 

 this class appear to be totally unlike such forms as the Oyster and 

 the Clam. However, the mantle fold and other essential structures 

 of the phylum are present. 



Phylum Arthropoda 



This phylum contains the greatest number of species. The dis- 

 tinguishing characteristics are: The body is made up of segments, 

 each segment being covered with a tough external skeleton com- 

 posed of CHITIN. Most of the segments bear a pair of appendages 

 which are jointed. The head bears one or more pairs of long, jointed 

 antennae. The digestive canal forms an inner tube, the body wall 

 forms an outer tube, and the coelom is the body cavity between 

 these two tubes. The symmetry is bilateral. The nervous system is 

 well organized and condensed into segmental ganglia with connect- 

 ing tracts. All segmental ganglia except the most anterior pair are 

 ventral to the alimentary canal. The hearts are modified dorsal or 

 segmental blood vessels. Two types of respiratory mechanisms are 

 found; gills attached to the bases of the walking appendages, as in 

 the Crayfishes, or modified as book lungs as in certain Spiders; and 

 TRACHEA, which are fine tubes that plumb the air throughout the 

 tissues, as in the Insects. There are five classes: 



Class I. CRUSTACEA (Fig. 57). Examples, Crayfish, Lobsters, and 

 Crabs. 



Class II. ONYCHOPHORA (Fig. 58). A form which in some im- 



