CLASSIFICATION 429 



oped somewhat in the Arthropodan direction. Lankester thinks their 

 evolution is as follows : 



Group Articulata 



1. Rotifera to Tardigrada 



2. Chaetopoda 



a. Proarthropoda (Peripatus) developing independently. 



b. Crustacea separate origin from Chaetopoda. 

 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 ob- 



vious bilateral symmetry, sometimes an eight-parted calcareous shell and 

 several pairs of gills. 



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. 



Class PELECYPODA ( ). Mollusks without 



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

 tle of two lobes. Clams, mussels. 



Class CEPHALOPODA ( ). 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. Cer- 

 tain terrestrial species with tracheae (Oniscidae sowbugs). 



Class MEROSTOMATA ( ). Fossil Arthro- 



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

 cephalothorax, 12-segments in abdomen, and pointed telson. Euryp- 

 terus. 



Class POECILOPODA ( ). Arthropoda with 



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

 with extremely long pointed telson. Limulus, king crabs. 



