266 ZOOLOGY. 
Articulata of Cuvier as a heterogeneous assemblage of 
forms embracing at least three branches of the animal king- 
dom, namely, the Vermes, 
Tunicata, and Arthropo- 
da. 
The Arthropoda are di- 
vided into six well-de- 
fined classes, #.¢., the 
f . Crustacea with two body- 
( \* regions, the head-thorax 
¢ 
Fig. 220.-Shrimp, Palemonetes vulgaris. and abdomen (Fig. 220), 
@, cephalo-thorax ; 6, abdomen. 
and breathing through 
the body-walls or by external gills; the Podostomata, which 
are marine and breathe by gills, while the remaining four 
classes breathe by intgrnal air-tubes and live on land. These 
are the Malacopoda, Myriopoda, Arachnida, and Insecta. 
Crass I.—Crustacea (Water-jleas, Shrimps, Lobsters, 
and Crabs). 
Gereral Characters of Crustaceans.—The typical forms 
of this class are the craw-fish, lobster, and crab, which the 
student should carefully examine as standards of comparison, 
from which a general knowledge 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 welt 
for the craw-fish, which abounds in the rivers and streams 
of the Middle and Western States. 
The body of the lobster consists of segments (somites, 
arthromeres), which in the abdomen. are seen to form a com- 
plete ring, bearing a pair of jointed appendages, which are 
inserted between the sternum and tergum, the pleurum not 
being well marked in the abdominal segments. The abdo- 
men consists of seven segments. One of these segments 
(Fig 221 D’) should be separated from the others by the stu- 
dent, m order to observe the mode of insertion of the legs. 
Each segment bears but a single pair of appendages, and it 
