ANIMALIA. INVERTEBRATA.
Remains of invertebrate animals occur in strata of every age, from the partially metamorphic and crystalline rocks of the Cambrian system to the deposits formed by the floods of last winter and the tides of yesterday. They are found in every country, from the highest latitude attained by Arctic voyagers to the extremities of the southern continents, and at the greatest elevation hitherto climbed in the Andes or Hima laya. If some classes—e.g., Tunieata, Acaleplue—seem not to be represented in stratified deposits, they are such as, either from the soluble or perishable tissues composing the entire frame, could not be expected to be fossilized under any con ceivable circumstances ; or from the same cause, are only not so recognisable at one of their metagenetic phases. Evidence of compound Hydrozoa--i. e., of the polypes which Ellis called " Corallinea "—and especially of the genus Campanularia, would show that the acalephal type and grade of organization had been manifested at the period of the formation of the strata containing such fossil Polypi. With the above seeming exceptions, every class of invertebrate animal is represented by fossil remains.
They consist of corals and shells, of the petrified skeletons of star-fishes and sea-urchins, of the hard coverings of crabs and insects, of the tracks and shelly- habitations of worms, and of impressions of surfaces, and casts of cavities, of organisms, retained by the matrix after the animals had perished.
The condition in which invertebrate fossils occur depends on the nature of the matrix and other accidental circum stances ; for while some are scarcely altered in composition, or even in colour, others are silicified or infiltrated with car bonate of lime. Some may be cleared by the action of acid
or exposure to the weather, and some require the chisel of the mason or the mill of the lapidary for the proper exhibition of their structure.
Multitudes of recent species are fossilized in the newer tertiaries whose history can be made out perfectly from living specimens ; but the number of these diminishes gradually in each older stratum, while the proportion of extinct forms is ever on the increase. No living specks more highly organized than a Rhizopod is found in the secondary rocks. Recent genera extend further back in time ; indeed a few may be recognised in strata of palTozoic age, shedding a light on the probable affinities and conditions of their associates. Many of the smaller groups of genera, called families, disappear in the secondary, and still more in the palozoic period, and are to a limited extent replaced by groups which no longer exist. But as to the larger groups of Protozoa and of true inverte brate animals, it may be affirmed that every known fossil belongs to some one or other of the existing classes, and that the organic remains of the most ancient fossiliferous strata do not indicate or suggest that any earlier and different class of beings remains to be discovered, or has been irretrievably lost, in the universal Metamorphism of the oldest rocks.