CLASS IV. CRUSTACEA: ORDER 10. PARASITA. 613 
ORDER 7. PHYLLOPODA. 
This order includes the Apodidce, in which the body is protected by a carapace which takes 
the form of a bivalve shell, and also the Branchipodidce, which are without carapace. One 
species, Branchipus stagnalis, is often found in cart-ruts. This also seems to be the place for 
a family of fossil Crustacea, the well-known Trilobites, Trilohita, of which vast 
numbers occur in some of the earlier strata of the earth's crust. Their general 
form is shown in the annexed figure of the Calymene Blumenhachii ; they pos- 
sessed well-formed, compound, faceted eyes, which are frequently well preserved 
in the fossil state. The body is usually divided into three regions, of which the 
first and last are commonly in the form of semicircular plates, while the middle 
portion exhibits distinct segmentation, and by its flexibility enabled the animal 
to double itself up in the manner of the common wood-louse. These animals are 
now quite extinct, although during the period of the deposition of the ancient 
strata in which their remains are found, they were almost the only representa- 
tives of the class Crustacea. 
THB CALTMBNE. 
ORDER 8. OSTRACODA. 
In this order, composed of animals generally of very minute size, the body, which strongly 
resembles that of the Copepoda, is always inclosed in a little bivalve shell, the feet and antennae 
being protruded between the lower edges of the valves. These little shells so closely resemble 
those of minute bivalve mollusca, that those of some of the larger species have actually been de- 
scribed by conchologists as the coverings of animals belonging to that class. The antennae are 
often curiously branched, and the hinder extremity is usually produced into a sort of tail, which 
is seen in constant action when the animal is in motion. 
This order forms two families — the Cypridce, in which the body is entirely inclosed within 
the shell, of which the genus Cypris is an example, and the DapUniadm^ in which the head is 
protruded beyond the shell. In the Polyphemus^ belonging to the latter, the head, which is 
large, is almost entirely occupied by an enormous eye, giving the creature a most singular ap- 
pearance. 
ORDER 9. COPEPODA. 
These are minute animals, with the body divided into segments. They appear to possess no 
distinct respiratory organs, and the ova are carried in sac-like 
organs attached to the abdomen of the mother. They occur 'in 
countless swarms in all waters, whether salt or fresh, and, minute 
as they are, one species is said to constitute the principal food 
of the Antarctic whale. The best-known form is the genus Cy- 
clops^ specimens of which may be found in every stagnant pool ; 
it is the type of the family Cyclopidae^ characterized by the pos- 
session of a single eye. The species Cyclops navicularis is found in stagnant fresh water in the 
Southern States. A closely-allied species is found in Lake Ontario. 
THE CYCLOPS. 
ORDER 10. PARASITA. * 
This order is composed of numerous small animals, which, in their young state, are furnished 
with distinct jointed limbs, antennae, and eyes, organs which either disappear completely or be- 
come greatly modified as the animals approach maturity, when they attach themselves to fishes 
or other aquatic animals, and pass the remainder of their existence as parasites. There are sev- 
eral families : one species, the Argulus foliaceus, is very common upon various European fresh- 
water fishes. The A. catostomi is found on the gill-covers of a species of sucker in Connecticut, 
and the A, alosce, on the gills of the alewife in Massachusetts. 
