ANNELIDA — EINDED WORMS — ARTHEOPODA. 
81 
vation. The case is not like a solid box, but is divided into 
a number of segments, separated as a rule by softer flexible 
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 Artliropoda (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, butterHies, 
beetles, and bugs ; Chilopoda or centipedes ; Diplopoila or 
millipedes ; Crustacea, including crabs, lobsters, sand-hoppers, 
wood-lice, barnacles, and water-lleas; 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 {Uuri/ptcrus, Fterij- 
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 tliat 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 winch 
the Classes successively became dominant during geological 
time. 
Gallery 
VIII. 
