CRUSTACEA. 
79 
Class I. — Crustacea {Water-fleas, Shrimps^ Lobsters, 
and Crabs), 
General Characters of Crustaceans. — The typical forms 
of this class are the craw-fish, lobster, and crab, which the 
student should carefully examine, as from tliem a general 
knoAvledge of the class, which varies greatly in form in the 
different orders, may be obtained. The following account 
of the lobster will serve quite as well for the craw-fish, 
which abounds in the rivers and streams of the Central and 
Western States. 
The lobster's body consists of segments, six of which in the abdo- 
men are seen to form a complete ring, bearing a pair of jointed ap- 
pendages. The abdomen consists of seven segments. One of these 
segments (Fig. 94, D') should be sep- 
arated from the others by the student, 
in order to observe the mode of inser- 
tion of the legs. Each segment bears 
but a single pair of appendages, and it 
is a general rule that in the Arthropods 
each segment bears but a single pair 
of appendages. The abdominal feet 
^^ 1 \i . 4 „ 41 Fig 95 — Mandible of the lobster, 
are called swimmerets; they are Homarus americanus: pal\ 
narrow, slender, divided at the end palpus, 
into tvvo or three lobes or portions, 
and are used for swimming, as well as in the female for carrying 
the eggs. The first pair are slender in the female (Fig. 94, ) and 
not divided, while in the male (Fig. 94, B $) they are much larger, 
and aid in reproduction. The sixth segment (Fig. 94, G) bears broad 
paddle-like appendages, while the seventh segment, forming the end 
of the body and called the telson," bears no appendages. It repre- 
sents the tergum alone of the segment. Turning now to the cephalo- 
thorax, we see that there are two pairs of antennae, the smaller pair 
the most anterior; a pair of mandibles with a palpus, situated on 
each side of the mouth; two pairs of maxillge or accessory jaws, 
which are flat, divided into lobes, and of unequal size; three pairs of 
foot-jaws (maxillipedes), which differ from the maxillae in having 
gills like those in the five following pairs of legs.* There are thus 
* The students can separate these limbs in a boiled lobster or cray-fish, and 
compare them with the Quts, He will find the exergise an interesting one, 
