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]]—
| Branch A. Phanerocephala. | |||||||||
| Sub-Order 1. Nereidiformia [= Errantia, auctt. + Ariciidae]. | |||||||||
| Family | 1. | Syllidae | see p. | [306] | Family | 8. | Amphinomidae | see p. | [318] |
| " | 2. | Hesionidae | " | [308] | " | 9. | Eunicidae | " | [318] |
| " | 3. | Aphroditidae | " | [309] | " | 10. | Glyceridae | " | [320] |
| " | 4. | Phyllodocidae | " | [313] | " | 11. | Sphaerodoridae | " | [320] |
| " | 5. | Tomopteridae | " | [315] | " | 12. | Ariciidae | " | [321] |
| " | 6. | Nereidae | " | [315] | " | 13. | Typhloscolecidae | " | [321] |
| " | 7. | Nephthydidae | " | [317] | |||||
| Sub-Order 2. Spioniformia. | |||||||||
| Family | 1. | Spionidae | see p. | [321] | Family | 4. | Magelonidae | see p. | [325] |
| " | 2. | Polydoridae | " | [323] | " | 5. | Ammocharidae | " | [325] |
| " | 3. | Chaetopteridae | " | [323] | |||||
| Sub-Order 3. Terebelliformia. | |||||||||
| Family | 1. | Cirratulidae | see p. | [325] | Family | 3. | Ampharetidae | see p. | [330] |
| " | 2. | Terebellidae | " | [327] | " | 4. | Amphictenidae | " | [330] |
| Sub-Order 4. Capitelliformia. | |||||||||
| Family. Capitellidae, see p. [331]. | |||||||||
| Sub-Order 5. Scoleciformia. | |||||||||
| Family | 1. | Opheliidae | see p. | [331] | Family | 4. | Scalibregmidae | see p. | [334] |
| " | 2. | Maldanidae | " | [332] | " | 5. | Chlorhaemidae | " | [334] |
| " | 3. | Arenicolidae | " | [333] | " | 6. | Sternaspidae | " | [335] |
| Branch B. Cryptocephala. | |||||||||
| Sub-Order 1. Sabelliformia. | |||||||||
| Family | 1. | Sabellidae | see p. | [336] | Family | 3. | Amphicorinidae | see p. | [339] |
| " | 2. | Eriographidae | " | [338] | " | 4. | Serpulidae | " | [339] |
| Sub-Order 2. Hermelliformia. | |||||||||
| Family. Hermellidae, see p. [341]. | |||||||||
Comparative Anatomy of the Polychaeta.
General Shape of the Body.—The majority of the Polychaeta have an elongated and very mobile body, like that of Nereis, consisting of an indefinite and usually of a considerable number of segments; a few, however, have a shorter body, with fewer segments, definite in number, for instance Aphrodite and Polynoë, which have thirty to forty segments; and some Hesionids, with only some seventeen to twenty segments.
In Aphroditidae and certain Amphinomidae the body is more or less oval in shape. In Lipobranchius and Sternaspis it is grub-like, short, and cylindrical, with rounded ends; in the former it is difficult to distinguish head and tail, or dorsal and ventral surfaces.
The segments composing the trunk may be all alike, or may constitute two more or less sharply marked regions, the thorax and abdomen, differing in the character of the chaetae, or in their arrangement, or in some other way, as in the Sabelliformia and the Capitelliformia.
As peculiar cuticular structures, the curious shields of Sternaspis, and of certain of the Maldanidae may be mentioned.