APPENDIX B 



649 



with a pair of two-branched ventral appendages. The group has been discussed in 

 Chap. XXVII. 



Class 2. The Crustaceans (Crustacea). Mostly aquatic; usually with gills; 

 chitinous exoskeleton usually stiffened with limy deposits into a carapace. Head 

 (of five fused segments) with two pairs of antennae, a pair of jaws, and two pairs 

 of maxillae; segmental appendages usually two-branched (biramous). This large 

 group of some 25,000 existing species includes marine, fresh-water, and terrestrial 



Fig. B.17. Some Paleozoic trilobites. Upper, the Silurian Dalmanites, swimming. The rest 

 are Devonian species. Lower left, Arctineurus; center, Calymene; right, Terataspis grandis, 

 which reached a length of 18 inches and is one of the largest known trilobites. {Courtesy 

 University of Michigan Museums.) 



types with a wide range in form and size. The principal groups are the Bran- 

 chiopoda (brine shrimps, fairy shrimps, water fleas, etc.), the Ostracoda (minute 

 bivalved crustaceans), the Copepoda (small forms important in the plankton), 

 the Cirripedia (goose barnacles and rock barnacles, Fig. 32.6), and the great 

 subclass Malacostraca, which includes by far the majority of crustaceans. Among 

 the malacostracans are included various shrimplike forms, the flattened isopods 

 (pill bugs, wood lice, and various free-living and parasitic aquatic forms), the 

 compressed amphipods (sand fleas, etc.), the mantis shrimps (Squilla, etc.), and 

 the decapods (true shrimps, prawns, lobsters, crayfishes, crabs). 



