MOLLUSCOIDEA.—
Group five, the Molluscoidea, contains two types; the Flustras or Sea-mats (Polyzoa) and the Lamp-shells (Brachiopoda). They are at first sight totally unlike; for the first-named are colonies of compound animals, and the second are simple, and enclosed between two valves. They show in common, however, a bilateral symmetry. The mouth is furnished with fine tentacles, or with spirally rolled hair-like or ciliated processes.
MOLLUSCA.—
The sixth group, the Mollusca, includes all shell-fish. They are soft-bodied, bilaterally symmetrical animals, without definite segments. The shells, on account of being formed of carbonate of lime on an organic basis, are often found preserved in fossiliferous strata.
ARTHROPODA.—
The seventh group, the Arthropoda, or joint-footed animals, are distinguished by their segmented, lateral limbs, and by having a body composed of a series of segments or somites. The body and appendages are usually protected by a horny covering, the ‘exoskeleton.’ The group of the Trilobites played an important part in the first era of the formation of the earth’s crust; whilst the other groups were more sparsely represented in earlier geological times, but became more and more predominant until the present day.
VERTEBRATA.—
The great group of the Vertebrata comes last, with its chief characteristic of the backbone structure, which advances in complexity from the Fishes to the Higher Mammals.
A Simplified Classification of the Vegetable Kingdom.