226 INVERTEBRATE ZOOLOGY 



Of the six pairs of cophalothoracic appendages, the first are 

 preoral, and the remaining five pairs, which are for walking, have 

 their bases modified as masticating organs. In addition to a pair 

 of lateral compound eyes, the cephalothorax bears a pair of 

 median ocelli. 



Members of the order Eurypterida have a small cephalothorax 

 and a large twelve-jointed abdomen. These forms flourished 



during past geological ages and in 

 structure seem to be intermediate 

 between the scorpions and the mem- 

 bers of the order Xiphosura, of which 

 Limulus, the horseshoe crab, is the 

 only living example. In Limulus, the 

 telson is long and spikelike. The 

 abdomen bears six pairs of appendages 

 of which the first forms a broad, flat 

 operculum which overlaps the follow- 

 FiG. 106.— Ventral view of ing five pairs of platelike appendages, 



trilobite larva of Limulus i (• i • i i -n tt 



Polyphemus. {After Kingdey) . ^ach of whlch bears a gill. UpOn 



hatching from the egg, the young 

 Limulus is said to be in the trilobite stage (Fig. 106) because 

 of its resemblance to the organisms of that group. 



Subclass Arachnida 



Scorpions, spiders, mites, harvest men, and some less commonly 

 known Acerata are grouped under a common subclass Arachnida. 

 Most of these are air-breathing forms in which the 

 cephalothorax bears one pair of pedipalps lateral or immediately 

 posterior to the mouth and a pair of preoral appendages termed 

 the chelicerae. In addition to these appendages, four pairs of 

 walking legs are just as characteristic of the arachnids as three 

 pairs are for the insects. As an exception to the foregoing, it 

 should be noted that as a rule young mites have but three pairs 

 of legs (Fig. 108 C), and in some gall mites only two pairs of 

 legs are found. 



True jaws are entirely lacking. Few arachnids swallow solid 

 objects. The chelicerae and pedipalps are frequently modified 

 for crushing the prey, and in some instances the bases of some of 

 the walking legs serve the same function. Usually, only the body 

 juices of the victim are taken into the stomach and this is accom- 

 plished by action of a muscular sucking stomach. 



