504 A POPULAR EXPOSITION OF THE 
dian rocks, to pass briefly in review the classification-characters of the 
leading zoological groups as recognised in existing Nature. Animal 
organi:ms appear to be constructed after five principal types: the so- 
called Protozoic type, the Radiated type, the Molluscous type, the 
Articulated type, and Vertebrated type. 
Prorozoa stand upon the extreme and oscillating limit of the 
Vegetable and Animal worlds. They include a series of Infusorial 
forms, in great part of vegetable origin, Sponges and Rhizopods. 
RaptaTep ANIMALS exhibit, at least in their typical forms, a radiated 
arrangement of their structural parts, as seen in the coral polyp, the 
sea-urchin, and the starfish. They are all aquatic, and chiefly marine. 
Mo.utuscovus ANIMALS, as the name implies, are soft-bodied, and the 
greater part secrete an external calcareous shell, as in the oyster and 
the snail. In some few, however, the shell is internal, as in the cuttle- 
fish ; and some again, as the common slug, are without a shell, or pos- 
sess merely the rudiments of one. ARTICULATED ANIMALS comprise 
insects, crustaceans (as the lobster, crab, &c.,) and other forms with 
usually a distinctly jointed body, covered in many instances by a 
hard integument or even by ashell. Finally, VerTEBRATED ANIMALS 
possss an internal skeleton, of which the principal and most persistent 
part is the vertebral column. They include fishes, batrachians, (as 
newts and frogs,) reptiles, birds, and mammals. 
Since the first creation of living things, representatives of each of 
these great types—that is to say, of the Radiated type, the Molluscous 
type, &c.,—probably peopled the earth in each and all of its varied 
periods of development; but hitherto, traces of vertebrated forms 
have escaped detection in the lowest fossiliferous rocks, fishes 
first appearing in Europe at the extreme top of the Upper Silurian 
deposits, and with us, in the Devonian strata. 
Protozoa.—This sub-kingdom includes: Inrusorta, SponGes, and 
RHIZOPODS. 
Inrusor1a.—These are microscopic organisms, for the greater part, 
if not wholly, of vegetable origin, although (as in the case of the well- 
recognized spores or earlier stages of development of many crypto- 
gams) possessing powers of locomotion. Recent Infusoria occur in al] 
waters in which decomposed matters are present, and they are fre- 
quently found also in clear running streams. Some are entirely soft- 
bodied, but others are protected by a calcareous, siliceous, or ferrugi- 
nous shell. The microscope has shewn that many bog-iron deposits, 
