NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 1 39 



derived from primitive aquatic scorpions cannot be maintained against 

 the evidence of close relationship between the Xiphosurida and the 

 Trilobita. The Arachnida, as invaders of the land, had to evolve 

 organs for aerial respiration, and the lamellate gills of their aquatic 

 progenitors borne on the abdominal appendages were structures 

 readily convertible into "lung books" by invagination into pockets of 

 the integument (see Lankester, 1885). In addition, however, tracheal 

 ingrowths of the body wall were developed in the Arachnida, as they 

 have been in nearly all the other terrestrial arthropods. 



75. — The Protomandibulata preserved the slender, polypodous, 

 centipedelike form of the primitive protarthropods, but they acquired 

 as a distinctive character a pair of jawlike feeding organs, the 

 mandibles, developed from the bases of the second postoral appen- 

 dages. Probably long before the evolution of the mandibles, the first 

 somite had been united with the prostomial acron to form a primitive 

 composite head, or protocephalon, bearing the acronal sensory organs, 

 the mouth, and the first pair of postoral appendages, which last 

 became a second pair of antennae. The two pairs of appendages 

 following the mandibles were reduced and modified to serve as acces- 

 sory feeding organs. The other appendages were probably all leglike 

 in form, as in modern centipedes, and were 7-segmented, since a 

 patella does not occur in the mandibulate branch of the arthropods. 

 The circulatory system still retained the basic structure of that of 

 the generalized annelids ; respiration probably was branchial, the gills 

 being carried on epipodite lobes of the coxopodites, as in the Trilobita ; 

 the nephridial organs were perhaps suppressed in most of the body 

 segments, but those that remained were of the onychophoran type of 

 structure. The reproductive organs were closed gonadial tubes open- 

 ing in each sex through a single pair of ducts formed from a pair 

 of coelomic sacs, but the segmental position of the genital openings 

 varied in different forms according to what particular pair of coelomic 

 sacs served as gonadial outlets. 



The primitive Protomandibulata probably inhabited both the water 

 and the land, since from them were early evolved the aquatic Crus- 

 tacea, while the main branch developed into the terrestrial Proto- 

 myriapoda, from which have descended the modern myriapods and 

 the Hexapoda. 



16. — That the Crustacea are derived from crawling, centipedelike 

 protomandibulate ancestors is attested by the retention in all the higher 

 forms of ambulatory appendages having the same structure as the 

 limbs of terrestrial arthropods. Many forms, however, have become 

 adapted in part or entirely to swimming by a modification of the 



