The POLYCHAETA are marine worms whose bodies are usually elongated and cylindrical; they either lead a free life, swimming in the open sea, or crawling along the bottom; or they pass their life in burrows or definite tubes of various kinds. Each segment is normally provided on each side with a single or a couple of bundles of chaetae, by means of which locomotion is effected. These, in the free-living forms, are carried at the ends of lateral muscular outgrowths of the body, known as "parapodia," which are practically limbs. The "head" of the worm generally carries eyes, and frequently more or less elongated tactile organs, the "tentacles" dorsally and "palps" ventrally. The foregut is frequently provided with a masticating apparatus in its anterior region, which is capable of protrusion; but this apparatus is absent in many burrowing and tubicolous forms. The sexes are separate, so that there is no such complicated system of generative organs as occurs in the Oligochaeta. The nephridia usually act as genital ducts. In the majority of cases the egg develops into a larva, the "Trochosphere," which leads a free life and undergoes a greater or less metamorphosis into the adult condition. The CLASSIFICATION OF POLYCHAETA adopted in this work is as follows:[312]— {258}BRANCH A. PHANEROCEPHALA. SUB-ORDER 1. _Nereidiformia_ [= _Errantia_, _auctt._ + _Ariciidae_]. Family 1. Syllidae see p. 306 " 2. Hesionidae " 308 " 3. Aphroditidae " 309 " 4. Phyllodocidae " 313 " 5. Tomopteridae " 315 " 6. Nereidae " 315 " 7. Nephthydidae " 317 " 8. Amphinomidae " 318 " 9. Eunicidae " 318 " 10. Glyceridae " 320 " 11. Sphaerodoridae " 320 " 12. Ariciidae " 321 " 13. Typhloscolecidae " 321 SUB-ORDER 2. _Spioniformia._ Family 1. Spionidae see p. 321 " 2. Polydoridae " 323 " 3. Chaetopteridae " 323 " 4. Magelonidae " 325 " 5. Ammocharidae " 325 SUB-ORDER 3. _Terebelliformia._ Family 1. Cirratulidae see p. 325 " 2. Terebellidae " 327 " 3. Ampharetidae " 330 " 4. Amphictenidae " 330 SUB-ORDER 4. _Capitelliformia._ Family. Capitellidae, see p. 331. SUB-ORDER 5. _Scoleciformia._ Family 1. Opheliidae see p. 331 " 2. Maldanidae " 332 " 3. Arenicolidae " 333 " 4. Scalibregmidae " 334 " 5. Chlorhaemidae " 334 " 6. Sternaspidae " 335 BRANCH B. CRYPTOCEPHALA. SUB-ORDER 1. _Sabelliformia._ Family 1. Sabellidae see p. 336 " 2. Eriographidae " 338 " 3. Amphicorinidae " 339 " 4. Serpulidae " 339 SUB-ORDER 2. _Hermelliformia._ Family. Hermellidae, see p. 341. COMPARATIVE ANATOMY OF THE POLYCHAETA.