The Principles of Biology, Volume 1 · Herbert Spencer
INVERTEBRATA.
I. APATHETIC ANIMALS. } } Cl. 1. Infusoria. } Do not feel, and move only by their Cl. 2. Polypi. } excited irritability. No brain, no Cl. 3. Radiaria. } elongated medullary mass; no senses; Cl. 4. Tunicata. } forms varied; rarely articulations. Cl. 5. Vermes. } 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.
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