PHYLUM ANNELIDA

Annelida are true worms with elongated bodies composed of a series of similar segments.

Order Polychaeta. These are marine worms of great abundance provided with many setae and typically with a variety of well-defined head structures such as eyes, tentacles, chitinous jaws, ciliated cirri, and so forth, which are modified in keeping with their habits of life and mode of feeding. They have a wide distribution horizontally and bathymetrically.

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For the most part they are benthic, either creeping or burrowing, as in Nereis, Glycera, and Arenicola, or sedentary in calcareous or fibrous tubes, as in Spirorbis and Sebella. Tomopteris is pelagic (fig. 228b). There are in all over 3500 species, nearly all marine.

Order Oligochaeta. These are earthworms, of which only a very few are marine, living near shore.

Class Echiuroidea. These are fleshy marine worms with only one or two pairs of setae. They are unsegmented or indistinctly segmented in the adult. They live in burrows in the mud and sand of the littoral zone. There are about 20 species.