Figure 12.24: A terrestrial arthropod, a species of woodlice. ( 20 )eggs. Others, such as woodlice, lay their eggs on land, although in damp conditions. In
other crustaceans, the females keep the eggs until they hatch into free-swimming larvae.
Six classes of crustaceans are generally recognized:
Table 12.3:Class Information
Branchiopoda Includes brine shrimp
Remipedia A small class restricted to deep caves con-
nected to salt water
Cephalocarida The horseshoe shrimp
Maxillopoda Includes barnacles and copepods
Ostracoda Small animals with bivalve shells
Malacostraca The largest class, with the largest and most
familiar animals: crabs, lobsters, shrimp,
krill, and woodliceCentipedes and Millipedes
Centipedes and millipedes belong to the subphylum Myriapoda, which contains 13,000
species, all of which are terrestrial, and which are divided among four classes. They range
from having over 750 legs (a species of millipede) to having fewer than ten legs. They have
a single pair of antennae and simple eyes.
They are most abundant in moist forests, where they help to break down decaying plant
material, although a few live in grasslands, semi-arid habitats, or even deserts. The majority
are herbivores, but centipedes are chiefly nocturnal predators.
Although not generally considered dangerous to humans, many from this group produce
noxious secretions, which can cause temporary blistering and discoloration of the skin. Cen-
tipedes are fast, predatory, and venomous. There are around 3,300 described species, ranging
from one tiny species (less than half an inch [about 12 mm] in length) to one giant species,
which may exceed 12 in (30 cm).
Most millipedes are slower than centipedes and feed on leaf litter anddetritus(loose mate-
rial, such as stone fragments, gravel, or sand, worn away from rocks). Around 8,000 species
have been described, although there may be as many as 80,000 or more species actually alive.
The third class, Symphyla, contains 200 species. They resemble centipedes but are smaller
and translucent. Many spend their lives in the soil, but some live in trees. The pauropods
are typically 0.5-2.0mm long and live in the soil of all continents except Antarctica. Over
700 species have been described, and they are believed to be closely related to millipedes.