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.= SERIES. | FORMS FOUND FOSSIL. ---------------------+---------------------------------- I.--THALLOPHYTA |Sea-weeds: as Corallines and | Calcareous Algae. | II.--BRYOPHYTA |Mosses, Liverworts. | III.--PTERIDOPHYTA |Fern-like plants, as Horse-tails, | Club-mosses and true Ferns. | IV.--PTERIDOSPERMEAE|Oldest Seed-bearing plants, | with fern-like foliage. | V.--GYMNOSPERMEAE |Plants with naked seeds, as Cycads | (Fern-palms), Ginkgo | (Maiden-hair Tree), and | Conifers (Pine trees). | VI.--ANGIOSPERMEAE |Flowering plants, as Grasses, | Lilies and all ordinary trees | and plants. =Characters of the Plant Series.= THALLOPHYTA.--