136 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



a mechanism of telescopic movement between successive body seg- 

 ments by the simple device of retaining nonsclerotized areas in the 

 posterior parts of the primary segments, thus establishing a secondary 

 segmentation in which the longitudinal muscles became interseg- 

 mental instead of intrasegmental in action. The sclerotized appen- 

 dages necessarily became segmented into individually movable parts, 

 and their movements became more specifically controlled by body 

 muscles inserted on their bases. The protarthropods retained the 

 annelid structure of the nervous system, and the independence of 

 the first postoral ganglia of the ventral nerve cords. The prostomial 

 appendages (antennules) assumed an anterior position by a forward 

 migration below the eyes, with the result that in the arthropod brain 

 the antennal lobes lie beneath the optic lobes, and the brain takes a 

 vertical position by contrast with the horizontal position of the ony- 

 chophoran brain. Lateral eyes of the compound type were first 

 developed in the Protarthropoda. Because of the origin of the Protar- 

 thropoda from Protonychophora, the protarthropods were equipped 

 with a series of nephridial organs like those of the Onychophora, and 

 their internal reproductive organs were of the onychophoran type. 

 The segmental relations of the genital ducts, however, were subject 

 to mutation, and the position of the gonopores was, therefore, dif- 

 ferent in different forms, as shown by the highly variable position 

 of the genital outlets in modern arthropods. 



The Protarthropoda, having an annelid ancestry, and being directly 

 derived from wormlike protonychophorons by a sclerotization of the 

 integument and a jointing of the appendages, could scarcely take on 

 other than a centipedelike form and structure, though they did not, 

 of course, have the composite head and other specialized features of 

 present-day myriapods. The number of body segments was variable, 

 and potentially large, since the production of new somites in the 

 zone of growth was not limited. The cephalic appendages (anten- 

 nules) were filamentous, the lateral eyes primitively compound. The 

 body appendages were probably all ambulatory legs with little differ- 

 entiation among them, each composed of seven segments. The 

 dactylopodites were provided with extensor and flexor muscles aris- 

 ing in the propodites. Aquatic forms probably had branchial epipo- 

 dites on the coxopodites. Perhaps the majority of the protarthropods 

 lived in shallow water near the ocean shore, where they inhabited the 

 bottom or aquatic plants, but probably also they occurred abundantly 

 in debris along the beach, and very likely some the them were to be 

 found in damp places on the land. The genital openings being on 

 specific body segments, propagation took place by sex mating, though 



