284 
COMPARATIVE ZOOLOGY. 
maxillae, which works like the “screw” of a propeller. 
The heart is a single oval cavity, and drives arterial blood 
-—a milky fluid full of corpuscles. The alimentary canal 
consists of a short gullet, a gizzard-like stomach, and a 
straight intestine. 
Crustaceans pass through a series of strange metamor¬ 
phoses before reaching their adult form. They also peri¬ 
odically cast the shell, or moult, every part of the integu¬ 
ment being renewed; and another remarkable endowment 
is the spontaneous rejection of limbs and their complete 
restoration. Many species are 
found in fresh water, but the class 
is essentially marine and carnivo¬ 
rous. 
Of the numerous orders of this 
great class we will mention only 
four: 
1. Cirripeds , distinguished by 
being fixed, by having a shelly 
covering, and by their feathery 
arms {cirri). Such are Barnacles 
(Lepas) and A corn-shells {Balarms)) 
so common on rocks and timbers 
by the sea-shore. 
2. Entomostracans , which agree 
in having a horny shell and no abdominal limbs; repre¬ 
sented by the little Water-fleas {Cyclops) of our ponds, and 
the Brine-shrimps ( Artcmia ), and many others. The King- 
crabs {Limulus) and the extinct Trilobites were formerly 
tradecapod. U. S. coast. 
