ARTHROPODA. i237 
the nervous system is often lost or modified. The blood-circu- 
latory system may be absent; but when it is present, it is 
placed dorsally (fig. 56), and consists of a true blood-system 
containing corpusculated blood, antl furnished with a contrac- 
tile cavity or heart. Respiration is sometimes effected simply 
by the general surface of the body, but there are generally 
special organs, adapted for breathing air either directly or 
through the medium of water. Jointed appendages are always 
present, and may be developed from any segment of the body. 
The Arthropoda are divided into four great classes—viz., the 
Crustacea (crabs, lobsters, &c.), the Avachnida (mites, spiders, 
and scorpions), the AZyriapoda (centipedes and gally-worms), 
and the Jwsecta (or true insects). These are roughly distin- 
guishable from one another by the following characters :— 
I, CRUSTACEA.—Animal more or less truly aquatic ; respiration by 
gills, or by the general surface of the body ; two pairs of antennze 
(feelers) ; locomotive appendages more than eight in number, borne 
by the segments of the thorax, and usually of the abdomen also. 
2. ARACHNIDA.—Respiration aerial, by pulmonary sacs, by air- 
tubes (trachez), or by the general surface of the body; head and 
thorax amalgamated ; antennze (as such) absent ; legs eight ; abdo- 
men without jointed appendages, 
3. Myriapopa.— Respiration by air-tubes (trachez) ; head distinct ; 
remainder of the body composed of nearly similar segments ; one pair 
of antennz ; legs numerous. 
4. InsEcTA.— Respiration by air-tubes (trachez) ; head, thorax, and 
abdomen distinct ; one pair of antennz ; three pairs of legs borne on 
the thorax ; abdomen destitute of limbs ; generally two pairs of wings 
on the thorax, 
