290 ^"^ ANIMAL KINGDOM 



Sz-^rnLcnt 



1 



z 



3 



4 



Ti^ilobita 



? 



Ante-nnSL 



Arachnomorpha. 



Chelicera. 



L<^g 



Crusta-c<z.eL 



First 

 eLnte-TLTia. 



Second 

 ant<z.nn.a. 



Mandible 



First 

 ma3<:illa. 



Second 

 maxilla. 



Labi at cL 



Antenna. 



Ma-ndibl<2. 



Ma>cilla 



Labium, (pair) 



Figure 16.1. Appendages of the first six segments in the four subphyla of the Ar- 

 thropoda. Except for the labium (lower right) only one member of a pair is shown. The 

 chelicera illustrates a chelate appendage, in which the next to last segment is prolonged 

 as a hand against which the last segment closes as a thumb. 



Crustacea and Labiata. The first includes only one class, Trilobita, now 

 extinct. The trilobites (Fig. 16.2) were marine, bottom scavengers with 

 the skeleton extended laterally to form a three-lobed shield. The second 

 segment bore a pair of antennae and all remaining segments bore 

 biramous (two-branched) limbs. The inner branch or ramus of each 

 limb was used for walking while the outer ramus apparently served as 

 a gill. The single base of each limb was enlarged medially as a toothed 

 jaw or gnathobase. Debris was chewed by this long row of gnathobases 

 as it was passed forward to the mouth. The abundance of their fossils 

 suggests that trilobites were dominant organisms of the Cambrian 

 period, over 500 million years ago. During the rest of the Paleozoic era 

 they were gradually replaced by the Crustacea and became extinct 225 

 million years ago. 



The subphylum Arachnomorpha includes a variety of both living 

 and extinct groups such as king crabs, eurypterids, scorpions, spiders and 

 mites. In these forms the second segment has no appendages. Those of 

 the third are chelate (tipped with pincers) (Fig. 16.1). This particular 

 pair of chelate appendages are small, located in front of the mouth, and 



