ARTHROPODA. 
287 
Cray-fish (Astacus), Lobster ( Homarus ), and Crab {Can- 
cer). Crabs differ from Lobsters chiefly in being formed 
for creeping at the bottom of the sea instead of swim- 
ming, and in the reduction of the abdomen or “tail” to a 
rudiment, which folds into a groove under the enormous 
thorax. They are the highest and largest of living Crus¬ 
tacea: they have been found at Japan measuring fifteen 
feet between the tips of the claws. 
Class, II. —Myriapoda. 
Myriapods differ from Crustaceans and Spiders in hav¬ 
ing the thorax merged in the abdomen, while the head is 
free. In other words, the body is divided into similar 
segments, so that thorax and abdomen are scarcely distin¬ 
guishable. They resemble Worms in form and in the 
simplicity of their nervous and circulatory systems; but 
the skin is stiffened with chitin, and the legs (indefinite 
in number) are articulated. The legs resemble those of 
Insects, and the head appendages follow each other in the 
same order as in Insects—eyes, antennae, mandibles, max¬ 
illae, and labium. They breathe by tracheae, and have two 
antennae and a variable number of eyes. 
There are two orders : 
1. Chilognatha , having a cylindrical body, each segment, 
except the anterior, being furnished with two pairs of legs. 
They are of slow locomotion, harmless, and vegetarian. 
The Thousand-legged Worm ( Julus) is a common repre¬ 
sentative. 
2. Chilopoda , characterized by having a flattened body 
composed of about twenty segments, each carrying one 
pair of legs, of which the hindermost is converted into 
spines. They have longer antennae than the preceding, 
and the mouth is armed with two formidable fangs con¬ 
nected with poisonous glands. They are carnivorous and 
active. Such is the Centipede ( Scolopendra , Fig. 259). 
