Class VIII.— ARTHROPODA (Joint-footed). 



Characters. — Bilateral symmetry. Segmented, with a tendency for 

 anterior segments to fuse to form a Head, and in some cases a Thoracic 

 region. Numerous jointed appendages (mostly), some of which are con- 

 verted into mouth organs {jaws) which move laterally (not vertically as in 

 Vertebrates). 



I. CEUSTACEA. 



Character's. — Crustacea have the characters of Arthropoda and in 

 addition the following: — They inhabit water. Breathe mostly by gills 

 {branchice). Body usually divided into head, thorax, and abdomen. Two 

 pairs of antennce. Locomotory appendages usually attached to abdomen. 

 Head and thorax often fused to form cephalo-thorax. 



SXJB-DIVISIOlSrS. 



(1) Entomostraca. — Crustacea, with slight amount of fusion of the 

 separate segments of the exoskeleton. Never more than three appendages 

 converted into jaws. Embryos make their appearance mostly at Nauplius 

 (unsegmented larva with two or three pairs of appendages) stage. 



1. Phyllopoda ( Branchiopoda, Brine Shrimps). — A sheU-like 

 carapace or bivalve shell is formed by a reduplication of the skin. 

 One pair of antenna (as in Ifauplius) form swimming organs. 

 e.g. Apus ; Daphnia. 



2. CoPEPODA. — No branchisB. One pair of antennas form 

 swimming organs, e.g. Cyclops. 



3. CiRRiPEDiA. — Retrograded, sessile forms. Hermaphrodite. 

 Attached by anterior end. Body enclosed in a sac-like mantle 

 formed by the skin, which may remain soft or become calcified. 

 Leave the egg as a Nauplius. Anterior region is either broad {e.g. 

 Balanus, acorn shell) or drawn out into a stalk {e.g. Lepas, barnacle). 

 Huxley defines an acorn shell as a crustacean which is fixed by its 

 head and kicks food into its mouth by its hind legs. 



4. OsTRACODA (Water-fleas). — Carapace forms a complete bivalve 

 shelly covering for the body. Abdomen imperfectly developed. 



(2) Malacostraca. — Almost always twenty segments (five cephalic, eight 

 thoracic, six abdominal, and one telson), and all but one bear appendages; 

 as many as six appendages may be converted into jaws. Larvee mostly set 

 free at a stage [Zoea stage] later than the Nauplius stage. 



1. PoDOPHTHALAMATA. — Eyes stalkcd. 



(ffl) Schizopoda. — Soft cephalo-thoracic shield. Thoracic 

 appendages biramous. 



(&) Decapoda. — Large cephalo-thoracic shield, covering aU 

 cephalo-thoracic segments. Three thoracic appendages modified 

 to form maxilipedes. Gills attached to thorax and appendages. 



