292 ^^^ ANIMAL KINGDOM 



and insects. Their second segment has antennae and the third segment 

 lacks appendages. The fourth has mandibles, the fifth has maxillae, and 

 on the sixth appendages comparable to maxillae are fused together to 

 form a lower lip, the labium, from which the subphylum takes its 

 name. All of the appendages are uniramous. This group apparently 

 arose on land, although so many insects have developed aquatic young 

 that labiates now challenge the supremacy of crustaceans among fresh- 

 water arthropods. 



A fragmentary record of the appearance and spread of these sub- 

 phyla is shown by their fossils. At the beginning of the fossil record, 

 585 million years ago, Cambrian seas already contained numerous 

 species of trilobites. Within 60 million years, before the end of the 

 Cambrian period, the seas also contained arachnomorphs and crus- 

 taceans. Trilobites never left the ocean, but arachnomorphs appeared in 

 fresh water by the Ordovician period (505 million years ago), and crus- 

 taceans followed by the Devonian (375 million years ago). Certain 

 arachnomorphs (scorpions) became terrestrial by the Silurian (425 mil- 

 lion years ago), leaving for us the oldest known terrestrial fossils. The 

 labiates appeared as a terrestrial group during the coal age (Pennsyl- 

 vanian period, 275 million years ago). Among the earliest of these are 

 winged insects, indicating that the air had already been conquered 50 

 million years before flying reptiles and 110 million years before birds 

 appeared. Terrestrial crustaceans exist today, but all of their known 

 fossils are of recent origin. Thus terrestrialism developed independently 

 at least three times within the phylum. The insects now form a dominant 

 terrestrial group, their myriad species scattered from the arctic to the 

 equator, from the swamps to the deserts. 



The phylum can be subdivided in other ways. Subphyla may be 

 omitted, and the phylum is then divided into seven or more classes. The 

 trilobites and arachnomorphs may be placed in one subphylum. The 

 trilobites have also been grouped with the crustaceans, and it is not un- 

 common to find all of the antennate groups in one subphylum. The 

 arrangement used here is a combination of views current in zoology 

 and paleontology. 



1 33. Class Crustacea 



Crustaceans have two distinguishing features, the two pairs of an- 

 tennae already described, and a nauplius larva (Fig. 16.3). This larva 

 has an externally unsegmented body, a simple, median eye, and only 

 three pairs of appendages, the first pair uniramous and the other two 

 biramous. Its mouth is ventral between the second and third pairs of 

 limbs and the anus is terminal. This minute creature floats in the water 

 feeding upon microscopic plants and debris. As the larva grows and 

 undergoes several molts, additional limbs appear on segments added 

 in front of the anus, and the organism gradually assumes its adult 

 shape. The uniramous limbs of the larva become the first antennae of 

 the adult, the first biramous limbs become the second antennae, and 

 the third pair of limbs become the adult mandibles. Since the additional 



