IO8 THE LIVING WORLD. 



during the Silurian(2) age, and have lived continuously until now, 

 probably with little change. 



Arlhropoda, This province again consists of three classes. 



1. Crustacea (crabs, lobsters, etc.). With the Crustacea we meet 

 for the first time a class of animals, a considerable portion of whose 

 development has occurred since the Silurian (2). Still the class is 

 an old one, and most of the lower orders were distinct at our earliest 

 record of fossils. The Cirrepedia (barnacles), Ostracoda (water fleas), 

 Amphipoda (sand fleas), Trilobita, and Euripteridca, as well as one 

 order (Phyllocarida) which seems to have been the ancestor of the 

 higher Crustacea, were all in existence during the Silurian (2). But 

 the trilobites and euripterids attained their culmination at that time, 

 and soon after disappeared in the Carboniferous (4). The higher 

 orders certainly appeared later. The Isopoda (sow bugs) and Phyllo- 

 poda first appeared in the Devonian (3). The shrimps and lobsters are 

 found in the Devonian (3) and Carboniferous (4), the crabs probably 

 are in the Carboniferous. Now since the crabs are undoubtedly derived 

 from the lobster group, and this group from the Phyllocarida above 

 mentioned, we have in these orders an instance where we can trace 

 by fossils the origin of the modern orders from the earliest lower types. 

 The Crustacea have always been abundant, and, with the exception of 

 a few orders that have disappeared, are as abundant to-day as ever 

 before. The higher orders indeed were never so diversified as at the 

 present. 



2. A rachnoida (spiders and scorpions). Under this head are found 

 the oldest air-breathing animals. The scorpions were in existence in 

 the later Silurian (2), and true spiders are found in the Carboniferous 

 (4). That these have been in existence during all the later ages is 

 therefore certain, though only scanty records have been found. 



Myriopoda (thousand legged worms). Being mostly land animals, 

 these have left scanty remains. They are found in the Devonian (3), 

 and doubtless appeared earlier. 



Insecta. Though the insects form about five sixths of the animals* 

 of the world to-day, their habits prevent their ready preservation as 

 fossils, and our history of the group is quite meagre. Some of the 

 lowest orders (cockroaches) were undoubtedly in existence in the Si- 

 lurian (2). During the whole Paleozoic (2-4), insects were abundant. 

 All of them had a greater or less resemblance to the cockroach, al- 

 though in the Devonian (3) and the Carboniferous (4), they took on 

 features that allied them to the higher orders of insects, the beetles, 



