ANNELIDA — EINGED WOEMS — AETHEOPODA. 81 



vation. The case is not like a solid box, but is divided into Gallery 

 a number of segments, separated as a rule by softer flexible Vlil. 

 skin. In primitive forms the whole body is divided into a 

 series of generally similar segments, each bearing a pair of 

 limbs ; but in later forms several segments fuse more or less 

 completely, especially at the head end of the body. The 

 limbs also are segmented and their segments united by 

 flexible joints, whence the name Arthropoda (jointed feet). 

 A more striking feature, however, is that though, in their 

 essential structures, all these limbs are organs of locomotion, 

 some at the front end of the body, around the mouth, are 

 used for seizing and biting food : the feet have become jaws. 

 In most arthropods that live in the water some limbs 

 behind the jaw-limbs have developed plates or plumes, which 

 serve as gills. Land arthropods breathe either by small 

 lung-sacks or by long tubes called tracheae, which open to 

 the air by holes, called stigmata, in the sides of the body- 

 segments. 



The great majority of arthropods now living are divided 

 into the following Classes : Insecta, including flies, butterflies, 

 beetles, and bugs ; Chilopoda or centipedes ; Diplopoda or 

 millipedes ; Crustacea, including crabs, lobsters, sand-hoppers, 

 wood-lice, barnacles, and water-fleas; and Arachnida, including 

 spiders, ticks, and scorpions. All these Classes are represented 

 by numerous fossils back to Palaeozoic times ; but many fossil 

 arthropods are not obvious members of any of these Classes. 

 Such are the trilobites, the Eurypterida i^Eiirijpterus, Ftery- 

 gotus, &c.), and the king-crabs, which last have persisted to 

 our own day. Certain resemblances between these forms 

 have led some writers to unite them in a single Class. It is 

 now generally admitted that the king-crabs and Eurypterida 

 are related to the Arachnida ; but they may still be con- 

 veniently distinguished as Merostomata. The trilobites were 

 perhaps allied to the Merostomata, and yet there are some 

 features in which they resemble Crustacea. It may therefore 

 be as legitimate as it is convenient to keep them apart as a 

 Class Trilobita. These Classes will now be considered in the 

 order in which the British specimens are arranged in the 

 Table-cases, namely : Trilobita (Cases 25, 24) ; Arachnida 

 (Cases 24, 23) ; Crustacea (Cases 23-20) ; Diplopoda, Chilo- 

 poda, and Insecta (Case 20). Setting aside the centipedes 

 and millipedes, this order may be justified as that in which 

 the Classes successively became dominant during geological 

 time. 



G 



