Phylum Arthropoda 489 



sexes are separate in all but a few forms ; externally the sexes are often 

 easily dififerentiated from one another. Mating habits, embryology, and 

 larval stages are highly variable. 



Classes of the Arthropoda. — The classes of this phylum are 

 separated from one another by the diverse arrangements of the body 

 regions, the arrangement of the appendages, and other specialized struc- 

 tures. 



Class I. Onychophora. Indistinctly segmented, with elongated bodies, short 

 antennae, stumpy legs present on all segments but the first and last. Entirely 

 terrestrial; found only in tropical moist areas. Example: Peripatus. 



Class II. Crustacea. Body divided into cephalothorax and abdomen, two 

 pairs of antennae, one pair of jaws, two pairs of maxillae, respiration by gills or 

 body surface. Abundant in nearly all habitats, including fresh-water, marine, 

 and terrestrial. Examples: Cambarus, a crayfish, Homarus, a lobster, Uca, the 

 fiddler crab, Balanus, a barnacle, and Daphnia, the water flea. 



Class III. Trilobita. Fossil forms which were abundant in the ocean during 

 the early Paleozoic Era. Body was divided into three regions: head, thorax, and 

 pygidium; two longitudinal furrows divided the body longitudinally. With a 

 single pair of long segmented antennae on the head. Example: Triarthrus, a 

 trilobite. 



Class IV. Merostomata. Body in two parts, the cephalothorax and ab- 

 domen ; the abdomen with appendages which form book gills for respiration, three- 

 jointed chelicerae, a pair of palps and four pairs of legs, no antennae. Exclusively 

 marine forms. Example: Limulus, the horseshoe crab; extinct eurypterids. 



Class V. Arachnida. Body divided into cephalothorax and abdomen, with 

 paired chelicerae, two pairs of palps, no antennae, four pairs of legs, respiration 

 by book lungs or tracheae, mostly terrestrial forms. Examples: spiders, scor- 

 pions, mites, daddy longlegs, ticks. 



Class VI. Pycnogonida. Body short with a minute abdomen, cephalothorax 

 with seven pairs of appendages, proboscis with suctorial mouth, entirely marine 

 forms. Example: Nymphon, sea spider (Fig. 171,o). 



Class VII. Pentastomida (Linguatulida). Wormlike forms with a larval 

 form similar to that of the mites, unsegmented body, elongate abdomen, short 

 cephalothorax, no circulatory, respiratory or excretory organs. All parasitic. 

 Examples: Porocephalus, Linguatula, the tongue worms (Fig. \7\,g). 



Class VIII. Tardigrada. With small rounded bodies, four pairs of unjointed 

 legs terminating in claws, no segments, no circulatory, respiratory, or excretory 

 systems ; all hermaphroditic. Found in both fresh and salt water and in wet moss. 

 Example: Macrobiotus, the water bear (Fig. 171, /t). 



Class IX. Diplopoda. Body elongate with segments fused into pairs, with 

 two pairs of legs per external segment, paired antennae, jaws, and maxillae on 

 head, sex openings on third segment, respiration by tracheae. Exclusively terres- 

 trial forms. Examples : Spirobolus, Fontaria, millipedes. 



