ANIMAL 511 vertebrata (those animals having a spinal col- umn), containing foui classes mammalia, birds, reptiles, and fishes. II. The mollusca. III. The articulata. IV. The radiata. The 3lasses are divided into 72 orders, and the latter into the different genera and species. The classification of Prof. Agassiz, founded on that of Cuvier, is one of the most modern, and in many respects one of the most instructive. It is as follows : Branch I. RADIATA. Class 1. Polypi : including the 2 orders, actinoids and hal- cyonoids. " 2. Acalephce : 8 orders hydroids (including siphono- phorse), discophorse, and ctenophowe. " 8. Echinoderms : 4 orders crinoids, asteroids, echi- noids, and holothurioids. Branch II. MOLUTBOA. Class 1. Acephala: 4 orders bryozoa (including the verti- celte), brachiopods, tunicata, and lainellibranchi- ata. " 2. Gasteropoda: 8 orders pteropoda, heteropoda, and gasteropoda proper. " 8. Cephalopoda: 2 orders tetrabranchiata and di- branchiata. Branch IK. ABTTOTLATA. Class 1. Worms : 8 orders trematods (including cestoids, planarke, and leeches), nematoids (including acan- thocephala and gordiacei), and annelides. . " 2. Crustacea: 4 orders rotifera, entomostraca (in- cluding cirripeds), tetradecapods, and decapods. " 8. Insects : 8 orders myriapods, arachnids, and in- sects proper. Branch IV. VERTEBRATA! Class 1. Myzontes: 2 orders myxiuoids and cyclostomes. " 2. Fishes -proper : 2 orders ctenoids (as the perch) and cycloids (as the cod). [This division will probably be considerably modified by its author.] " 8. Ganoids : 8 orders caelacanths, acipenseroids, and sauroids ; and doubtful, the siluroids, plectogna- thi, and lophobranches.
- 4. Selachians : 3 orders chimfErae, galeodes, and ba-
tides. " 5. Amphibians: 8 orders csecilise, ichthyodi, and anoura. " 6. Reptiles: 4 orders serpents, saurii, rhizodontes, and testudinata. " 7. Birds : 4 orders natatores, gralte, rasores, and in- sessores (including scansores and accipitres). " 8. Mammalia: 8 orders marsupialia, herbivora, and carnivora. As to the chemical composition of animals, probably only 17 out of the 64 simple elements now known, or at most 19, enter into their structure. These are : Oxygen, Sulphur, Magnesium, Chlorine, Hydrogen, Phosphorus, Sodium, Fluorine, Carbon. Calcium, Potassium, Silicon, Nitrogen, Iodine, Manganese, Bromine. Copper, Lead, Iron, These elements, variously combined, form numerous compounds, termed the immediate principles of animal structures, of which in the mammalia there are about 90. Of these, some are of mineral origin, as water, common salt, and phosphate and carbonate of lime. Some are formed within the bodies of animals by disassimilation, as urea, uric acid, and crea- tine ; and others still are obtained from vege- table and animal food for the nutrition of the tissues, as albumen, caseine, musculine, and fat, and hence exist as constituent elements in the latter. These immediate principles unite to form the tissues of which the parts and or- gans of animals are constituted. These are in the vertebrata as follows : 1. Epithelial Tissue: A. Epidermis and its modifications nails, hoofs, horns, scales, and shells. B. Hah- and its modifications bristles, wool, and feathers. 2. Elastic Tissue, its properties much resembling those of gum elastic. 8. White Fibrous Tissue, in tendons, ligaments, &c., very strong and almost totally inextensible. 4. Osseous and Dental Tissues, in bones and teeth. 5. Areolar Tissue, connecting the various organs to- gether. 6. Adipose Tissue, otherwise called fat. 7. Cartilage, in the joints, those of tbp ribs, Ac. 8. Muscular Tissue, the source of motion. 9. Nervous Tissue, the seat of sensation and intelligence. 10. Membranes, cutaneous (sKin), mucous, and serous. 11. Glands, the secretory organs. 12. Vessels, the blood vessels and the lymphatics. The tissues of animals are developed direct- ly from the vital fluid, the blood. This in all the vertebrata is red, from the presence of minute cells containing a colored fluid, and which are called the blood corpuscles. In the invertebrate animals no such cor- puscles exist, and therefore the blood is color- less. Hence the division of animals into the red-blooded and the white-blooded. The blood of each animal in the central parts of the body has its peculiar natural temperature, that of man being 98 to 99 F. The tem- perature of all animals lower in the scale than birds is lower than that of human blood, and hence all these are called cold-blooded, while birds and the mammalia are termed warm- blooded animals. The fecundity of animals also varies inversely with their elevation in the scale. While mammals produce from 1 to 8 or at most 10 young at a time, a tench produces 38,000 and a mackerel 646,000 eggs; and Leeuwenhoek professes to have counted 9,384, - 000 eggs in a single codfish. Some of the mam- mals are, however, very prolific. Pennant cal- culates that the descendants of a single pair of rabbits would, without interference, amount in four years to 1,274,840. But external circum- stances exert a powerful influence in this re- gard. For instance, the pigeon in its wild state broods but twice a year, but when do- mesticated six, and sometimes even nine times. In the latter case, a single pair would in four years produce 14,762 descendants according to Stillingfleet, and according to Linnaus, over 18,000. The astonishing fecundity of some of the animalcules has already been illustrated. The mammalia alone bring forth their young alive, the duration of gestation being as follows in the following species : elephant, 20 months and 18 days; rhinoceros, 9 months; horse, 11 months; ass, 12 months; cow, 9 months; reindeer, 8 months ; buffalo, 12 months; sheep and goat, 5 months ; foxes and wolves, 5 months; Greenland whale, about 10 months. Obviously there must be a correspondence of the structure of an animal to its habits and functions. For example, a carnivorous animal must have great strength and powers of loco- motion, enabling it to overtake, seize, and de-
