II. SENSITIVE ANIMALS. } Feel, but obtain from their sensations } only perceptions of objects, a Cl. 6. Insects. } sort of simple ideas, which they are Cl. 7. Arachnids. } unable to combine to obtain complex Cl. 8. Crustacea. } ones. No vertebral column; a brain Cl. 9. Annelids. } and mostly an elongated medullary Cl. 10. Cirripeds. } mass; some distinct senses; muscles Cl. 11. Conchifera. } attached under the skin; form symmetrical, Cl. 12. Mollusks. } the parts being in pairs. VERTEBRATA. { Feel; acquire preservable ideas; III. INTELLIGENT ANIMALS. { perform with them operations by which { they obtain others; are intelligent in Cl. 13. Fishes. { different degrees. A vertebral column; Cl. 14. Reptiles. { a brain and a spinal marrow; distinct Cl. 15. Birds. { senses; the muscles attached to the Cl. 16. Mammalia. { internal skeleton; form symmetrical, { the parts being in pairs. Passing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to classifications which recognize another order of facts--those of development. The embryological inquiries of Von Baer led him to arrange animals as follows:-- I. Peripheric Type. (RADIATA.) _Evolutio radiata._ The development proceeds from a centre, producing identical parts in a radiating order. II. Massive Type. (MOLLUSCA.) _Evolutio contorta._ The development produces identical parts curved around a conical or other space. III. Longitudinal Type. (ARTICULATA.) _Evolutio gemina._ The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis. IV. Doubly Symmetrical Type. (VERTEBRATA.) _Evolutio bigemina._ The development produces identical parts arising on both sides of an axis, growing upwards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures. Recognizing these fundamental differences in the modes of development, as answering to fundamental divisions in the animal kingdom, Von Baer shows (among the _Vertebrata_ at least) how the minor differences which arise at successively later embryonic stages, correspond with the minor divisions.