432 General Biology 



From Crustacea by separate origin 



a. Myriapoda 



b. Insecta 



c. Arachnida. 



Paleontologists, such as Walcott, the specialist on trilobites and 

 worms, derive all arthropoda classes by separate lines from trilobites. 



Phylum MOLLUSCA ( ). Triploblastic, bilat- 



erally symmetrical (symmetry often obscured) unsegmented animals 

 with a coelom, a muscular foot and usually a shell. Mollusks. 



Class AMPHINEURA ( ). Mollusks with 



obvious bilateral symmetry, sometimes an eight-parted calcareous shell 

 and several pairs of gills. Chitones and Chaetoderma. 



Class GASTROPODA ( )'. Mollusks with a 



head and with bilateral symmetry usually obscured by a spiral shell of 

 one piece. Snails. 



Class SCAPHOPODA ( ). Mollusks with 



conical tubular shell and mantle. Dentalium. 



Class PELECYPODA ( ). Mollusks without 



a head, with bilateral symmetry, a shell of two lateral valves and a 

 mantle of two lobes. Clams, mussels. 



Class CEPHOLOPODA ( ). Mollusks with 



distinct bilateral symmetry and a foot bearing eyes and divided into 

 arms, usually with suckers. Cuttlefishes, octopods. 



Phylum ARTHROPODA ( ). Triploblastic, 



bilaterally symmetrical, segmented animals with usually more or less 

 dissimilar somites, a coelom very much reduced, paired jointed ap- 

 pendages, and chitinous exoskeleton. 



Class CRUSTACEA ( ). Arthropods breathing 



by means of gills, two pairs of antennae, crayfishes, crabs, shrimps. 

 Certain terrestrial species with tracheae (Oniscidae — sowbugs). 



Class MEROSTOMATA ( ). Fossil 



arthropoda of gigantic size (2 meters in length), without antennae, short 

 cephalothorax, 12 segments in abdomen, and pointed telson. Eurypterus. 



Class POECILOPODA ( ). Arthropoda with 



large shield-shaped cephalothorax, abdomen with six pair lamellate legs, 

 with extremely long pointed telson. Limulus, king crabs. 



Class LINGUATULIDA ( ). Parasitic 



arthropoda (Pentastomidae) of worm-like build, body with metameric 

 circular muscles, two pairs of hooks in region of mouth, mouth without 

 mandibles. Affinities uncertain. 



