SEGMENTED SCHIZOCOEIS 



231 



(mites and ticks) cephalothorax and abdomen are 

 fused together; cephalothorax has first pair of ap- 

 pendages with claws (chelicerae), one pair of pro- 

 jecting appendages (pedipalps), and four pairs of legs, 

 but no antennae; cephalothorax and abdomen typi- 

 cally without obvious external segmentation. 



CLASS MEROSTOMATA (Merostomates) 



Diagnosis: lateral eyes two, each composed of 

 many individual perception units grouped into a com- 

 pound eye; body covered by a shell (including an ex- 

 tensive, solid anterior portion, the carapace), ab- 

 dominal appendages bearing specialized respiratory 

 structures (gills), abdomen terminated by an elon- 

 gate, dagger-like structure; only Chelicerata with ab- 

 dominal legs, five or six pairs. 



Subclass EURYPTERIDA (Sea Scorpions) 



Diagnosis: all extinct, Ordovician to Pennsyl- 

 vanian; only vaguely scorpion-like, mostly small 

 marine predators, but one to 7 feet long, the largest 

 known fossil arthropod; external skeleton (carapace) 

 covering cephalothorax but not expanded (Figure 

 14.6). 



Figure 14.6 Subclass Eurypterida, Eurypterus, o Silurian sea scorpion. 



Subclass XIPHOSURA (King, or Horseshoe, Crabs) 



Diagnosis: mostly extinct, Cambrian to Recent; 

 cephalothorax covered by an arched, expanded 

 carapace of horseshoe outline; Limulus to 20 inches 

 long (Figure 14.7). 



Only about five species of the genus Limulus, the 

 king or horseshoe crabs, are now living, inhabiting 

 the Atlantic Coast from Maine to Florida, the West 

 Indies, and the eastern shores of Asia. These species 



Figure 14.7 Subclass Xiphosurida, timu/us, a horseshoe or king crab, 

 marine, grow to 18 inches long. 



are very similar in appearance, all giving the im- 

 pression of something out of the remote past. Their 

 appearance is not deceptive, because they are close 

 copies of horseshoe crabs that lived almost a half 

 billion years ago. Therefore, the present horseshoe 

 crabs truly are living fossils. 



CLASS PYCNOGONIDA (Sea Spiders) 



Diagnosis: mostly minute, but to about 20 inches; 

 vaguely spider-like; four simple eyes; walking legs 

 usually eight (also 10 or 12); without respiratory 

 or excretory systems (Figure 14.8). 



Figure 14.8 Class Pycnogonida, Pycnogonum, a sea spider, mostly 

 minute to small. 



The sea spiders are a secretive and semisedentary 

 group of marine animals. Pycnogonids are infre- 

 quently seen, because their color matches their sur- 

 roundings and their habit of feigning death when 

 disturbed makes them inconspicuous. Most of their 

 activity is confined to crawling slowly over seaweeds 

 and attached animals. Although sea spiders are 

 most frequently encountered on hydroids they are 

 also found on anemones, algae, or under rocks, as 

 well as in the ocean depths. 



The sexes are usually separate, and the male car- 

 ries the eggs until they hatch. Depending on the 

 particular species, the eggs either hatch to a four- 



