272 PRINCIPLES OF ANIMAL BIOLOGY 



and knife handles. On the debit side of the ledger, the shipworm Teedo 

 (Fig. 234, a mollusk, not a worm) bores' into wharves and shipping and 

 does considerable damage. 



Class I. Amphineura. Mollusca with obvious bilateral symmetry, sometimes an 

 eight-parted calcareous shell and several pairs of gills. (Fig. 230.) 



Order 1. Polyplacophora Order 2. Aplacophora 



Class II. Gastropoda. MoUusks with a head and with bilateral symmetry usually 

 obscured by a spiral shell of one piece. Snails. (Fig. 232.) 



Subclass I. Streptoneura Subclass II. Euthyneura 



Order 1. Aspidobranchia Order 1. Opisthobranchia 



Order 2. Pectinibranchia , Order 2. Pulmonata 



Class III. Scaphopoda. Mollusca with conical tubular shell and mantle. 

 Class IV. Pelecypoda. MoUusks without a head, with bilateral symmetry, a 

 shell of two lateral valves and a mantle of two lobes. Clams, mussels. (Fig. 231.) 



Order 1. Protobranchia Order 3. Eulamellibranchia 



Order 2. Filibranchia Order 4. Septibranchia 



Class V. Cephalopoda. MoUusks with distinct bilateral symmetry and a foot 

 bearing eyes and divided into arms usually with suckers. Cuttlefishes, octopods. 

 (Fig. 233.) 



Order 1. Tetrabranchia Order 2. Dibranehia 



Phylum 9. Arthropoda. — Members of this phylum have jointed 

 bodies and jointed legs. Their skeletons are composed of a horny mate- 



FiG. 235. — A centipede. Fig. 236.— A beetle. 



(From Hegner, "College Zoology," The Macmillnn Company.) 



rial on the outside of the body. This horny shell is burst and shed at 

 intervals, and replaced by a new skeleton beneath, as the animal grows. 

 Examples of arthropods are crayfishes, shrimps, centipedes (Fig. 235), 

 insects (Fig. 23(5), and spiders (Fig. 237). They are triploblastic and 

 ))ilaterally symmetrical. The blood system includes sinuses, which are 

 merely spaces among the organs, into which the arteries open. The 

 coclom is much reduced in size. 



The number of differcmt kinds of arthropods is almost unbelievably 

 great. More known species belong to this phylum than to all other phyla 

 combined. About half a million have been described, but the number 



