8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



food and bringing it to the mouth. These legs later become struc- 

 turally modified to serve specifically as feeding organs. The labrum, 

 w^hen developed as a preoral lobe, must have served to stop the food 

 passed forward where it could be taken into the mouth. The ap- 

 pendages utilized as feeding organs, however, differ in different 

 arthropod groups. 



In modern Onychophora the claws of the first pair of legs have 

 been converted into a pair of flat "jaws" working in a vertical plane 

 in front of the mouth. The ancient trilobites had no jaws or other 

 special mouth parts, but the legs had spiny lobes on the inner sides 

 of the coxae, by which probably food was grasped and passed for- 

 ward to the mouth. In the ancestors of the chelicerate arthropods the 

 first pair of postoral appendages became small pincerlike organs, the 

 chelicerae, from which this group gets its name. In another early 

 arthropod group the coxae of the second postoral legs were developed 

 into a pair of jaws, the mandibles, working in the transverse plane, 

 while the rest of the limb was reduced to a palpus and usually elimi- 

 nated. Members of this group became the Mandibulata (crustaceans, 

 myriapods, and insects) characterized by the possession of mandibles. 

 The following pair of legs, or generally two pairs, were then modified 

 as accessory feeding organs, known as the first and the second 

 maxillae. In most of the Mandibulata the segments of these gnathal 

 appendages were combined with the protocephalon in the adult head. 

 In the anostracan and syncarid Crustacea, however, the gnathal seg- 

 ments remained as an independent group between the protocephalon 

 and the thorax. In those crustaceans having a maxillary carapace 

 united with the thorax, the gnathal segments were thereby anchored 

 to the thorax, leaving the protocephalon as the functional head. 



It is evident, therefore, that cephalization of the gnathal segments 

 has taken place independently in different arthropod groups, since the 

 tracheate mandibulates cannot be supposed to have been derived from 

 any crustacean having the same type of syncephalon. Among the 

 Crustacea the head of the isopods and amphipods most resembles 

 the insect head, but it includes the segment and appendages of a fifth 

 segment, that of the first maxillipeds. The head of the chilopods has 

 the same segmental composition as that of the insects, as has also 

 the symphylan head. In the pauropods, however, according to Tiegs 

 (1947), only one maxillary segment is contained in the head, and 

 probably the same is true of the diplopods. 



Just when in the ancestry of the insects the gnathal appendages 

 were modified for feeding and their segments added to the primitive 

 head we cannot know, since all known fossil insects appear to have 



