Branchial, Pedal Animals—Crustacea.
Conchozooid Ancyliozoa.
3507. The Crustacea are Worms with horny rings to the body, jointed feet and tentacula, which mostly breathe by branchiæ.
They correspond to the Conchozoa, or Shell-animals, and divide therefore into three orders, each of these including six families.
Here belong the Crabs, Wood-lice, and Spiders.
In the lowest forms the head, thorax, and abdomen are blended together, and the back mostly covered with a great horny scute—Mussel-like Insecta.
In the next place the thorax and abdomen admit of being clearly distinguished, both by their form as well as by their appendages, which are much larger on the thorax, as in the Cray-fish.
Then follow cylindrical forms with numerous feet, appended to uniform rings, but having a head free and distinguished by tentacula, maxillæ, and eyes—Wood-lice.
The Wood-lice bear a resemblance to the air-breathing Insects, although all parts of the body still pass over or blend gradually into each other.
In the Arachnida or Spiders aerial respiration takes place, and their body enters into relations with the more highly organized Insects, since the head becomes small, the abdomen large, thick, and short.