274 COMPARATIVE ZOOLOGY. 
hairs: this is the organ of hearing."* The gills, twenty on 
a side, are situated at the bases of the legs and inclosed 
in two chambers, into which water is freely admitted, 
in fact, drawn by means of a curious valve at the outlet 
which works like the “screw” of a propeller. The heart 
is a single oval cavity, and drives arterial blood—a dusky 
fluid full of corpuscles. The alimentary canal consists of 
a short gullet, a gizzard-like stomach, and a straight intes- 
tine. 
Crustaceans pass through a series of strange metamor- 
phoses before reaching their adult form. They also peri- 
odically cast the shell, or molt, 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 carnivorous. 
No natural classification of this varied group has been 
discovered. It will be convenient to divide it into four 
orders : 
1. Cirripeds, distinguished by being fixed, by having a 
shelly covering, and by their feathery arms (czr7rz). Such 
are Barnacles (Zepas) and Acorn-shells (Lalanus),so com- 
mon on rocks and timbers by the sea-shore. 
2. Entomostracans, which agree in having a horny shell 
and no abdominal limbs; represented by the little Water- 
fleas (Cyclops) of our ponds, the King-crabs (Limulus), 
abounding on every sea-coast, and the extinct Trilobites. 
The abdomen of the King-crab is reduced to a mere 
spine, the appendages about the mouth are used for loco- 
motion, and the eyes are smooth. 
3. Tetradecapods, small, fourteen-footed species; as the 
Wood-louse, or Sow-bug (Onzscus), so common in damp 
places, and the Sand-flea (Gammarus), seen by the sea- 
side in summer. 
4. Decapods, having ten legs, as the Shrimp (Cran- 
