2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



tained ; and many modern animals still feed in this manner. It was left 

 to evolution to produce accessory mouth structures for grasping, tear- 

 ing, crushing, or chewing that would enable their possessors to get 

 food in more concentrated form from plants or from the bodies of 

 other animals. Since such organs are entirely different in the different 

 groups in which they occur, it is clear that they have been independ- 

 ently developed. Thus we find in the polychaete worms a pair of 

 eversible pharyngeal hooks serving as jaws, in the mollusks a rasping 

 apparatus, in the sea urchins a complex apparatus with a set of 

 movable prongs surrounding the mouth, in the mandibulate arthropods 

 a pair of jaws fashioned from the bases of a pair of legs, and in the 

 vertebrates jaws derived from gill arches. 



The arachnids come from an ancestral line that never acquired 

 organs for mastication, and even today they have no true jaws. The 

 ancient trilobites probably were mud eaters; though they had plenty 

 of legs, the legs were not structurally differentiated for special pur- 

 poses, as in modern arthropods. The leg bases, it is true, were pro- 

 vided with strong, spiny mesal processes, but the latter did not meet 

 along the midline of the body, and could have had little use as feeding 

 organs other than perhaps that of stirring up the mud from which 

 the animals obtained their food. Likewise, true jaws have not been 

 developed in the Xiphosurida, though the first appendages of these 

 animals have taken the form of a pair of pincers, the chelicerae, 

 which serve for grasping and are said to be used for putting food 

 into the mouth, and the coxae of the next five pairs of appendages 

 are provided with large, spinous lobes, more highly developed than 

 those of the trilobites, but still not adapted for efficient mastication 

 of food. The mandibulate arthropods, however, have finally produced 

 from the coxae of the second postoral appendages a pair of strong 

 biting and chewing jaws. 



Chelicerae are characteristic appendages of the Xiphosurida and 

 the Arachnida. It is probable, therefore, but not a necessary assump- 

 tion, that the arachnids and the xiphosurids inherited their chelicerae 

 from some common progenitor. However, in the possession of che- 

 licerae and legs, the primitive arachnids were well equipped for 

 predatism and for terrestrial life; but, being without masticatory 

 organs, they were forced to subsist on the liquids they could obtain 

 from their prey. A liquid diet requires an ingestion pump, and, 

 with all arachnids, a highly developed sucking apparatus constitutes 

 the essential part of the feeding mechanism. Further structural 

 evolution related to the feeding function of the Arachnida, therefore, 

 should logically be in the direction of furnishing an efficient means 



