THE MAIN LINES OF ANIMAL EVOLUTION 



Figure 48. A Trilobite, Triarthrus becki. A, ventral view. B, section through a tlioracic 

 segment. C, a thoracic leg. a, endopodite; b, exopodite; c, protopodite with gnathobase. 

 (After Beecher, from Borradaile and Potts, "The Invertebrata." ) 



( Chapter 3 ) . Almost all crustaceans are aquatic, but there are some ter- 

 restrial species, such as the sowbugs. Like the trilobites, the crustaceans 

 are represented in early Cambrian rocks, and it is not improbable that 

 they arose in pre-Cambrian times. 



The Myriapoda, including the millipedes, centipedes, and their allies, 

 may have been derived from a crustacean progenitor by the reduction of 

 the exoskeleton and the loss of the gills. The latter were replaced by a 

 system of tracheae, small tubes which carry air to the tissues of the body, 

 thus permitting direct respiration. While not well represented in the fossil 

 record, yet they are known from rocks as old as the Devonian. Insects 

 probably also arose in Devonian times or earlier, but good fossils are first 

 seen in rocks of Pennsylvanian age. They may have been derived from 

 myriapods, or they may have come directly from crustacean progenitors. 

 Whichever ancestry they may have had, the tracheal system is always 

 present and is undoubtedly one of the important adaptations which has 

 permitted successful invasion of the land. The body became divided into 

 three well-defined regions, head, thorax, and abdomen. The appendages 

 of the head segments are all specialized either for sensation or for eating. 

 The thorax bears three pairs of walking legs. Dorsally, it bears two pairs 

 of wings, but one or both pairs may be lacking. The abdominal append- 

 ages are all suppressed. The immense variety of insects which has been 

 produced is indicated by the number of known species, approximately 

 660,000, or more than the combined numbers of species of all other living 

 groups. And the numbers of individuals of many of these species are truly 

 immense. 



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