THE TRILOBITA AND OTHER ARTHROPODA 215 



PHYLUM: ARTHROPODA. 



Metamerically segmented animals with jointed appen- 

 dages, the anterior somites fused into a head. 



CLASS : CRUSTACEA. 



Usually aquatic, and breathing by gills borne on the 

 appendages, with one or two pairs of antennae. 



SUB-CLASS : Trilobita. Classification already given. 



SUB-CLASS: Entomostraca. A rather heterogeneous 

 group of primitive Crustacea, all passing through the 

 stage of a Nauplius larva, with only three pairs of 

 appendages. Chief orders Phyllopoda, Ostracoda, 

 Cirripedia. 



SUB-CLASS: Malacostraca. With 19 somites (or in 

 Phyllocarida a few more) the head of 5 (bearing two pairs 

 of antennae, i of mandibles, 2 of maxillae), the thorax of 

 8 with walking limbs (of which the first i, 2, or 3 may 

 be fused with the head). Chief orders Phyllocarida, 

 Isopoda, Decapoda (with sub-orders Macrura and 

 Brachyura). 



CLASS: ARACHNIDA. 



Without antennae. Somites 18 or fewer. 



SUB-CLASS : Merostomata. Marine, breathing by 

 gills. Orders- -Xiphosura, Eurypterida. 



SUB- CLASS : Arachnida (sensu stricto}. Terrestrial, 

 breathing by lungs or tracheae. Chief orders 

 Scorpiones, Araneae. 



CLASS: MYRIOPODA. 



With numerous similar somites. Terrestrial, breath- 

 ing by tracheae. 



