CLASSIFICATION OF ANIMALS. 822 CLASSIFICATION OF ANIMALS. Grade 1. Intestinal Animaxs. Cycle 1. Digcsiirc Animals (Kadiata). Class 1. Infusoria (Stomacli Aniiiuils). Class 2. Polypi (Intestine Animals). Class 3. Aoali'phac (Lacteal Animals). Cycle II. Circnlative Aiihiuils (>l(>llusks). Class 4. Acophala (Biauriculate Animals). Class 5. Gastropoda (Uniaurieulate Animals). C'la.ss 6. Cephalopoda (Bicardial Animals). Cycle III. U'espirative Animals (Articulata). Class 7. Worms (Skin Animals). Class 8. Crustacea (Branchial Animals). Class 9. Insects (Tracheal Animals). At the time that the School of Natural Phi- losophy was thriving, another school had arisen in Germany, which was rapidly accumulating a new set of facts. This was the School of Em- bryologists, which seems to have arisen under the inlluence of DiJllinger and such men as Eathke, Van Beneden, Von Baer, Ki'dlikcr. and Vogt as disciples. Von Baer early got the idea that the ontogenetic develoimient of animals is a recapitulation of their phylogenetic history; and, in conformity with his cmbryological inves- tigations, he proposed the following classifica- tion of animals: I. Peripheral Type (Radiata). II. Massive Type (Molhisca). III. Longitudinal Tyi^e (Articulata). IV. Doubly Symmetrical Type ( Vertebra ta ) . Thus, Von" Baer. with his classification based on embryological principles, and Cuvier with his founded on comparative anatomy, arrived at very similar general conclusions; viz. that ani- mals are built (m fonr general plans, fall into four general types. In the end the system of Cuvier triumphed over that of the Natural Phi- losophers. The later advances in the classifica- tion of animals have been mainly made in filling gaps and correcting errors. The number of types has been increased. The Protozoa have become a distinct type, Coelenterata and Echinodermata have been separated ; likewise Arthropoda and Annelida, and the old group Mollusca has been broken up. The earlier classifications, founded mainly on external resemblances and gi'osser ana- tomical likenesses, were then greatly improved by the refinement of anatomical technique — by the use of the microscope, by the study of em- bryology, by the distinction between analogy and homology, by a knowledge of the fertility of animals in cross-breeding, and by the study of fossil-forms. With the discovery of so many intermediate forms we are coming back toward the views of the Natural Philosophers, who maintained that unity rules throughout the animal kingdom, as well as in all others; that intermediate forms connect the various types, and that there is no sharp line of demarcation to be drawn between the various groups of ani- mals. Two systems which have_ had great in- fluence, especially in directing the study of the difTerent invertebrates, were those of Louis Agassiz and Huxley. Agassiz retained flie four types of Cuvier. Most of the Protozoa he con- sidered to be alg,T. and such forms as the Vor- ticella he thought would be foiuid to be closely related to the Bryozoa. Following is an outline of Agassiz's arrangement, from his Essay on Classification (Boston, 18.59) : I. Radiata. ( 1 ) Polypi : Actinoids and ITalcyonoids. (2) Acalephse: llydroids, Discophorse, arid Ctenophone. (3) Eiliinoderms: Crinoids, Asteroids, Echi- noids, ilolotlunioids. II. Mollusca. ( 1 ) Acephala : Bryozoa, Brachiopods, Tuni- cata, Laniellibranehiata. (2) Gastropoda: Pteropoda, Heteropoda, and Gastropoda. ( 3 ) Cephalopoda : Tetrabranchiata and Di- branchiata. III. Aiticulata. ( 1 ) Worms : Trematodes, Nematodes, Anne- lids. (2) Crustacea: Rotifera, Entomostraca, Te- tradeeapods, and Decapods. (3) Insecta: llyriapods, Arachnids, and In- sects pro]jer. Huxley arranges the invertebrates in nine sec- tions, as follows: Section I. Monera (Foraminifera; Heliozoa) ; Kadiolaria, Protoplasta, Gregarinid;p, Catallacta, Infusoria (Opalinina, Ciliata, Flagellata, Tenta- culifera) . Section II. Porifera, Hydrozoa, Coralligena (Ctenopliora) . Section III. Turbellaria, Rotifera (Nemato- rhyncha ) , Trematoda, Cestoidea. Section IV. Hirudinea, Oligochfeta, PolychKta, Gephyrea. Section V. Crustacea, Araehnida (Pycnogo- nida, Tardigrada, Pentastomida), Myriapoda, Insecta. Section VI. Polyzoa, Brachiopoda, Lamelli- branchiata, Odonto])hora. Section VII. Echinodermata. Section VIII. Tunicata. Section IX. Groups of uncertain place: Peri- patidea, Myzostomata. Enteropneusta. Ch.Tptog- natha, Nematoidea, Physemaria, Acanthocephala, Dicyemida. '•bur knowledge," says Huxley, "of the anat- omy, and especially of the development of tlie Invertebrata, is increasing with such prodigious rapidity, that the views Of taxonomists in re- gard to the proper manner of expressing that knowledge by classification are undergoing, and, for some tiine to come, are likely to undergo, incessant modification. To the beginner, who is apt to make the mistake of looking upon classification as the foundation and essence of moriiho!og', instead of what it really is, the superstructure and outcome thereof, this state of things is distressing. Every handbook pre- sents him with a difTerent system of classifica- tion: and he may, not unnaturally, dcs[)air of finding any stability in science, the most general results of" which are capable of being stated in such diU'ercnt ways. If, however, the student will attend to the facts which constitute the subject-matter of classification, rather than to the modes of generalizing them which are ex- pressed in taxonoinic systems, he will find that, however apparently divergent these systems may be, they have a great deal in common." It has seemed desirable to dwell somewhat on the history of the classification of the Inverte- brata. because invertebrates have been the bat- tling-ground, the bone of contention, with sys- tematists since the time of Linna-us. The classi- fication of vertebrates was early agreed ipon; indeed, four of the six classes now accepted have come down to us from Aristotle's time.
