656 - Heredity and Evolution 



jointed poison claw. A tracheal respiratory 

 system is present (Fig. 32-30). 



Class 5. The D/p/opoda (the Millipedes, or 

 "Thousand-legs"). These elongate, often 

 brightly colored, terrestrial Arthropoda have 

 a cylindrical body, consisting of a distinct 

 head, with one pair of antennae; a short 

 four-segmented thorax, with one pair of legs 

 per segment; and a many-segmented abdo- 

 men, with tivo pairs of legs per segment. A 

 tracheal system is present, but there are no 

 poison claws (Fig. 32-30). 



Class 6. The Onychophora (Literally, Claw- 

 Bearing). This small group of primitive 

 wormlike Arthropoda consists of only 70 scat- 

 tered species, mainly of the genus Peripatus 

 (Fig. 32-30). These forms do not have a dis- 

 tinct head or other body subdivisions; and 

 the segmentation is not very evident, except 

 for the serial arrangement of the stubby 

 claw-bearing legs. The anterior end bears a 

 pair of short antennae and small horny jaws; 

 and the exoskeleton is relatively thin and 

 weak. Respiration is through internal air 

 tubes that seem to represent a primitive type 

 of tracheal system. Cilia, which are absent in 

 all other Arthropoda, are found in the gono- 

 ducts of Peripatus. 



Peripatus and related forms are interesting 

 because they seem to be surviving deriva- 

 tives of an ancient pre-Cambrian stock that 

 was transitional between the Annelida and 

 Arthropoda. In the list of resemblances to the 

 Annelida are the relative simplicity of the 

 eyes and digestive tract; the presence of cilia 

 and nephridia; and the poorly developed 

 appendages. On the other hand, the dorsally 

 placed heart, the tracheal tubes, the reduced 

 coelomic cavity, and the fact that the jaws 

 represent modified appendages are all fea- 

 tures suggestive of Arthropoda. 



Class 7. The Trilobita. The trilobites (Fig. 

 32-31) constitute a group of about 2000 ex- 

 tinct marine species. Trilobites reached a 

 peak of abundance in the Cambrian and De- 

 vonian periods, but died out, mainly in the 

 Carboniferous period. Some fossil specimens, 

 including larval stages, are exceedingly well 



Fig. 32-31. Trilobite fossil, from the Devonian period. 

 Note the two longitudinal furrows that divide the body 

 into three sections— one in the center and one on each 

 side. (By permission, from General Biology, by Wells 

 and Wells. McGraw-Hill Book Co., Inc.) 



preserved and these reveal much of the orig- 

 inal structure, internal as well as external. 

 As the name trilobite suggests, the body is 

 divided by two longitudinal furrows into 

 three lobes, one in the center and two at the 

 sides (Fig. 32-30). There is a distinct head, 

 bearing one pair of compound eyes (usually); 

 one pair of slender antennae, and four pairs 

 of biramous (two-branched) leglike append- 

 ages. A number (up to 29) of short separate 

 segments are present in the thorax; but the 

 abdominal segments are fused, forming a 

 caudal plate. Each of the segments, except 

 the last abdominal, bears a pair of bristle- 

 fringed biramous appendages. The average 

 size was about three inches, but one species 

 attained a length of more than two feet. The 

 best specimens found so far are from a de- 

 posit of Ordovician shale near Rome, New 



