It more interesting or important to the geologist can be inserted here. THE ANIMAL KINGDOM. I. INVERTEBRATES. I. PROTOZOA.—Animals simple in structure and usually minute in size, with bodies composed of a structureless jelly-like substance (sarcode) which, in some cases, secretes siliceous or calcareous needles or shells which serve to protect them. It is only these hard parts which have any chance of being preserved as fossils. CLASS i. RHIZOPODS, having generally a calcareous shell or siliceous Foraminifera—having usually a calcareous shell pierced with fine Radiolaria-marine creatures with radial siliceous skeleton, which together. They occur in vast numbers on some parts of the seafloor, where their remains form a siliceous ooze (Thalassicolla, Polycistina, p. 84). CLASS ii. INFUSORIA—protozoa living chiefly in fresh water, and having a definite form enclosed within an external membrane, and usually with a mouth and anus. From their perishable nature these animals are not met with in a fossil state. II. SPONGIDA (sponges), chiefly marine forms possessing an internal skeleton of horny fibres or of calcareous or siliceous spicules. The horny sponges are illustrated by the common sponge of domestic use which is the skeleton of a Mediterranean genus, composed of a close network of horny fibres. Such forms are too perishable to be looked for as fossils. The siliceous sponges secrete minute siliceous spicules which are dispersed in a network of sponge-fibres, sometimes in a glassy framework of six-rayed spicules (Hexactinellida). The calcareous sponges, as their name implies, secrete carbonate of lime as the substance of which their spicules consist (see Fig. 186). III. CŒLENTERATA (zoophytes), radially symmetrical animals with a body composed of cells arranged in an outer and an inner layer enclosing a body-cavity. Hydrozoa, including the fresh-water Hydra, and the marine jelly-fishes, millepores, Campanularia, Sertularia, etc. Most of these animals Ctenophora-spherical or cylindrical Medusæ, including the Venus Girdle Actinia or sea-anemone is an example; but it is an exception to the Rugosa (Tetracoralla), the older forms of coral in which the calcareous partitions are arranged in multiples of four with transverse partitions [Zaphrentis, Cyathophyllum, Amplexus, etc., Figs. 122, 137, 147]. Alcyonaria (Octocoralla), including Alcyonia, Pennatula, and Zoantharia (Hexacoralla), including the more modern forms of IV. ECHINODERMATA—animals possessing usually a symmetrical fivefold grouping of parts, and enclosed in a skin which is strengthened by hard calcareous granules, spicules, or close-fitting plates. Crinoidea-globular or cup-shaped, with jointed arms, and usually fixed by a jointed calcareous stalk. Most of the Crinoids are now extinct. Among the living forms are Pentacrinus, Rhizocrinus, Bathycrinus, and Comatula (see Figs. 149, 165, 173). Allied to the Crinoids are the extinct Cystideans (Fig. 123) and the Blastoids (Fig. 150), found in Palæozoic formations. Asteroidea (star - fishes).—The parts of these animals most readily preserved as fossils are the calcareous plates which run along the five rays of the star. These have been found in the marine deposits of many geological periods (Fig. 123). In the Brittle Stars (Ophiuroidea) the arms are flexible, cylindrical, and quite sharply marked off from the central disc. Echinoidea (sea-urchins), spherical, heart-shaped, or disc-shaped, with a usually immovable skeleton of calcareous plates which encloses the body like a shell and bears calcareous movable spines. They comprise the regular echinoids (Cidaris, Echinus, etc., Figs. 148, 174) and the irregular echinoids either compressed into the form of a shield (Clypeaster) or of a heart-shape (Spatangus, Fig. 187). Holothuroidea-worm-like elongated animals, with a leathery body in which the calcareous secretion is confined to isolated particles, scales, or spicules. These calcareous bodies have been found abundantly in the Carboniferous system, and are the only evidence of the existence of this division of the echinoderms at so ancient a period. Comparatively few V. VERMES, Comprising the various forms of worms. of these animals occur in the fossil state. Many of them are worms or flukes living in the intestines or other parts of the body. The only important class to the student of geological history are the annelides or segmented worms. Errantia, free-swimming predaceous sea-worms. The only hard parts of these creatures capable of surviving as fossils are the horny jaws which have been met with in some numbers even in ancient geological formations. But as many of the species live in and crawl over mud they leave behind them in their burrows and trails evidence of their presence. Such markings remain abundantly in many ancient rocks (Fig. 124). Tubicolæ sedentary worms, living within tubes within which they can withdraw for protection. This tube may remain as the only permanent relic of their existence. Sometimes it is a leathery substance; in other cases it consists of grains of sand or other particles cemented by a glutinous secretion, or of solid carbonate of lime. The most familiar example of this Order is the Serpula which may so frequently be seen encrusting dead shells thrown up upon the beach. Oligochata-earthworms and aquatic worms—are not found as fossils. But the common earthworm is an important agent in mixing the soil and bringing up its fine particles within reach of rain and wind (p. 18). VI. ARTHROPODA (Articulata). These differ from the worms in having jointed appendages attached to the body, which serve as organs of locomotion. They possess a tough chitinous skin which usually becomes hardened by the deposit of calcareous matter. The articulate animals are divided into four great classes as follows: i. Crustacea-chiefly aquatic forms with two pairs of antennæ and numerous paired legs. They include the Phyllopods, remarkable for their compressed bivalve shell which is frequently found in the fossil state (Fig. 126); the Ostracods, small forms enclosed in a bivalve shell, and with seven pairs of appendages, the minute shells being abundant in the fossil state (Cypris); the Cirripedes or barnacles, so commonly seen encrusting shore rocks; the Amphipods; the Isopods; the Decapods, which are either macrurous (long-tailed), as in prawns and lobsters (Fig. 178), or brachyurous (short-tailed), as in sea-crabs and land-crabs. A remarkable group of extinct Crustaceans is comprised in the Order Eurypterida (Fig. 135). The Xiphosura are still found living in the form of Limulus or King-crab; but they date back to the Carboniferous period. The earliest forms of Crustaceans belong to another extinct Order, the Trilobites (Figs. 118, 125, 136, 151). ii. Arachnida-air-breathing arthropods, with two pairs of jaws and four pairs of ambulatory legs, including mites, spiders, and scorpions. Some of these animals (scorpions) have chitinous integuments which resist decomposition and have been abundantly preserved in the rocks (pp. 215, 256, 273). iii. Myriapoda, including the chilopods or centipedes, feeding entirely on animals which they bite and kill with their poisonous secretion; and the chilognaths or millipedes and galley-worms which live in damp places and feed on vegetable and dead animal matters. iv. Insecta. Among the orders of insects of most interest in geological history are Orthoptera, with two usually unequal pairs of wings (earwigs, cockroaches, praying-insect, grasshoppers, locusts, crickets, booklice, termites or white ants, ephemeridæ or mayflies, dragonflies). Neuroptera, insects with wings in which the nervures form a network (Corydalis, camel-neck flies, ant-lions, phryganidæ or spring-flies). Hemiptera, including lice, cochineal insect, plant-lice, cicadas, bugs, water-bugs, water-scorpions. Diptera, with large glassy front wings, including the various kinds of flies, such as the house-fly, dung-fly, gad-fly, gnat, gall-fly, and flea. Lepidoptera, butterflies and moths. Coleoptera, beetles, the most durable parts of which are the horny wing-covers (elytra), so often to be found in woods and peatmosses. They include lady-birds, stag-beetles, tiger-beetles, etc. Hymenoptera, with four membranous wings, having few nervures, comprising ants, wasps, bees. VII. MOLLUSCOIDA-under this division may be grouped the Tunicaries, Polyzoa, and Brachiopoda. i. Tunicata, sea-squirts-simple or compound, fixed or free organisms which have been named from the leathery integument within which they are enclosed. Though some of them abstract carbonate of lime from sea-water, they present no hard parts for fossilisation, and the class is not known in the fossil state. ii. Polyzoa (Bryozoa), sea-mats and sea-mosses-composite animals, each enclosed in a horny or calcareous case, and united into colonies which are generally attached to some foreign body and often resemble plants in outer form. The calcareous colonies form durable objects which have been abundantly preserved as fossils. Polyzoa are met with among the oldest fossiliferous formations and still abound in the present sea. The common lace-like Flustra, so frequently to be seen encrusting the fronds of sea-weeds or dead shells, is a familiar example of them. Among the fossil forms (many of which have long been extinct) some of the most important genera are Fenestella ("lace-coral," Fig. 152), Polypora, Retepora, Glauconome, Hippothoa, Heteropora, Fascicularia. iii. Brachiopoda, lamp-shells-molluscous animals, having bivalve, calcareous, or horny shells, one valve placed on the back, the other on the front of each individual, and taking their name from two long ciliated arms which proceed from the sides of the mouth and create the currents that bring their food. They are grouped in two Orders: (1) The Inarticulata, in which the two valves are not united along the hinge line (Lingula, Fig. 119, Discina, Crania); and (2) the Articulata, in which the two valves are hinged together with teeth (Terebratula, Rhynchonella, Figs. 127, 138, 153, 160, 202). The brachiopods attained their chief development during the earlier periods of geological time, and are now represented by comparatively few living forms. The shells are equal sided, but the ventral is usually larger than the dorsal valve, and is prolonged into a prominent beak, by which it fixes itself, or through which the pedicle passes whereby it is attached to the seafloor. The following are characteristic genera : Terebratula (still living), Stringocephalus (Devonian), Thecidium (Trias to present time), Spirifera (chiefly Palæozoic), Atrypa (Paleozoic), Rhynchonella (Lower Silurian to present time), Pentamerus (Silurian), Orthis, Strophomena, Productus (Palæozoic), Leptana (Palæozoic to Lias), Crania, Discina, Lingula (from early Paleozoic to present time). VIII. MOLLUSCA―animals with soft bodies, inclosed in a muscular envelope which is usually covered with a strong calcareous shell. These hard shells are durable objects, and when covered up in sediment remain for an indefinite period as evidence of the existence of the animals to which they belonged. The great abundance of the mollusca also in the sea and in terrestrial waters gives a peculiar value to their remains. They (with the Brachiopoda) furnish by far the most valuable data to the geologist for the identification and comparison of marine sedimentary deposits of all ages. They are divided into the following classes : i. Lamellibranchiata-ordinary bivalves like the cockle, mussel, and Arcadæ, including among other genera Arca, Cucullaa (Fig. 139), Trigoniada- Trigonia (Figs. 175, 188), [Myophoria, Fig. 166, Unionida-Unio (river-mussel), Anodon, [Anthracosia]. Chamide-Chama, [Diceras, Requienia] [Hippuritidæ, Rudistes-Hippurites, Radiolites, Caprina, Caprotina, all confined to the Cretaceous system, Fig. 189]. |