CRUSTACEA. 139 
merely giving the names and leading characters of these in an 
appendix. It has also been thought advisable to invert the 
usual order here adopted, and to commence with the considera- 
tion of the highest sections of the class first. 
ORDER DeEcAPpopA.—The Crustacea included in this order 
derive their name from the fact that they all possess five pairs 
of legs (Gr. deka, ten; fodes, feet). They belong to a large 
section known as the “stalk-eyed” Crustaceans, from the fact 
that the eyes are supported by long, movable stalks. They 
include the lobsters, shrimps, cray-fish, crabs, hermit-crabs, 
and other forms, and are the most highly organised and most 
familiar of the whole class of the Crustacea. They are divided 
into three very well marked groups or tribes, all of which can 
be exemplified by well-known British species. 
A, Macrura.—The name of Macrura (Gr. makros, long; and 
oura, tail) is given to those ten-footed Crustaceans which have 
a long and well-developed tail or abdomen. Amongst these 
are the lobster, shrimp, prawn, and cray-fish, of which the lob- 
ster may be selected as a good typical example. 
In the lobster (fig. 63) the body is at once seen to be com- 
posed of two parts, familiarly called the “head” and “tail.” 
The so-called head is covered by a great shield termed the 
“carapace” (fig. 63, ca), and it is in reality the cephalothorax, 
being composed of the amalgamated segments which belong to 
the true head and to the thorax. The so-called tail is really 
the abdomen, and it is composed of a number of segments 
which are not immovably united together, as in the cephalo- 
thorax, but are movably jointed together. The various append- 
ages of the animal are arranged in pairs on the under surface 
of the body; and where the segments are completely amalga- 
mated (as in the cephalothorax), their existence may neverthe- 
less be determined by the presence of the successive pairs of 
appendages. The first segment of the head carries a pair of 
compound eyes, made up of a number of simple lenses aggre- 
gated together, and supported upon long and movable eye- 
stalks. Behind these come two pairs of jointed organs of 
touch,’ which are known as the “antennz.” The front pair is 
much smaller than the hinder pair, and they are known re- 
spectively as the “lesser antenne,” or “antennules,” and the 
“great antenne.” Behind these, again, comes the mouth, 
