Sipunculida

SIPUNCULIDA

Peanut worms

This is a small phylum of about 330 species of tough, unsegmented segmented worms with a short bulbous body and an elongate, tubular anterior process (the introvert), which can be squeezed out by muscular contraction, or retracted beack within itself. When disturbed the animals retract the anterior to assume the shape of a short club or even of a peanut kernel. The mouth at the tip of the introvert is surrounded by a frill of short tentacles, which gather detritalfood particles. These marine worms live shallowly in the sand or mud, nestling in empty shells, the holdfasts of kelps, about rocks or boring into rocks. Most are relatively large animals ranging in size from 2 mm to 72 cm.

Characteristics:

1. Symmetry bilateral. Triploblastic. No somites.

2. Body usually bluntly cylindrical posteriorly with a narrow anterior anterior retractileportion (introvert)bearing fine chitinious papilae and ending in a circle of short, hollow, cilliated tentacles.

3. Body wall of slightly roughened cuticule, epidermis, dermis with glands and sense organs and three layers of muscle: circular, oblique and longitudinal.

4. Mouth centered in tentacles, digestive canal slender(with ciliated groove) extending to posterior end and spiraled back on itself. The anus is dorsal, near the base of the introvert.

5. Coelom large and undivided. Peritonieum ciliated, coelomic fluid reddish(hemerythrin) , with several kinds of corpuscles.

6. No true circulatory organs but a tentacular system of dorsal and ventral vessels with ring sinus below the tentacles.

7. Nephridia (brown tubes) , two or none, opening near anus.

8.Nervous system of dorsal bilobed ganglion near tentacles with connectivesto single ventral unsegmented nerve chord, many lateral nerves.

9. Sexes seperate, but alike. No permanent gonads. Sex cells develop in the tissues on the retractor muscles of the introvert and the gametes escapes through the nephridia.

10. Larva usually a trochophore (no trace of somites).