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Lab 5: Annelida (mainly Class Polychaeta)

Phylum Annelida (= ringed): segmented worms

In this course we will not cover other important worm phyla, namely Platyhelminthes or flatworms (including planarians, liver flukes, and tapeworms), Nemertea (ribbon worms), and Nematoda (roundworms). However, the parasitology course covers these phyla in splendid detail. Annelids are triploblastic (= 3 germ layers)protostome coelomates with an organ-level organization. The annelid body is bilaterally symmetrical and vermiform (= worm-like), and metameric (= consisting of a series of repeating segments). The gut is complete (with both a mouth and an anus; food transport is 1-way), the circulatory system is closed, the nervous system is well-developed and cephalized (sensory structures tend to be located predominantly on the head), and nephridia comprise the excretory structures. Marine annelids have a trochophore larva. The phylum Annelida includes three classes, Polychaeta, Oligochaeta, and Hirudinea.

Class Polychaeta. Polychaetes, predominantly marine animals, comprise about 60% of all annelid species. A polychaete has a large internal body cavity (coelom) divided by well-defined septa. Most species have parapodia, lateral extensions of the body wall that promote gas exchange and often serve a locomotory function, as well. Parapodia typically have many setae (= bristle-like hairs). Polychaetes are either errant (homonomous, actively hunting predators) or sedentary (heteronomous, tube-dwelling or burrow-dwelling deposit or detritus feeders).

Examine carefully the demonstration slides of Nereis sp.

1. late larva: Identify segments, eyes, parapodia, setae (Brusca, fig. 34, p. 426).

2. parapodium: Identify notopodium, neuropodium, superior and inferior ligulae, dorsal cirrus, ventral cirrus, setae, acicula (see Brusca, fig. 5, p. 390).

Examine live Nereis virens

1. Does the body undulate in a serpentine fashion?

2. Do the parapodia on opposite sides of the body move synchronously?

3. How do all the parapodia on the same side of the body more? Simultaneously?

4. Do all the longitudinal muscles of each segment contract at the same time or are the muscles of only one side contracted at any one time?

5. How are the longitudinal muscles arranged and how do they opperate during locomotion in Nereis?

If you have a dissection available, observe it for the following internal anatomy: pharynx, esophagus, esophageal cecum (= esophageal gland), nephridia, intestine, septa, dorsal vessel, ventral vessel.

Examine carefully the preserved specimens on demonstration. Be sure you can tell errant polychaetes from sedentary polychaetes. Know the definitions of homonomous and heteronomous and how these terms apply to polychaetes.

Class Oligochaeta. Members of this class are terrestrial burrowers, such as the common earthworm, or freshwater animals. Oligochaetes lack parapodia and have only a few setae. They have fewer and less elaborate head appendages than do polychaetes. Oligochaetes are homonomous and have a distinctive clitellum, a group of modified segments used in reproduction.


Examine the pair of demonstration slides of cross sections of Lumbricus and Nereis. Be able to distinguish between them. (See Brusca, pp. 392-393.)

In the Lumbricus slide, identify the circular and longitudinal muscle, the typhlosole, dorsal blood vessel, and the cuticle.

In the Nereis slide, identify circular, longitudinal and oblique muscles, the dorsal blood vessel, the ventral vessel and ventral nerve cord, the intestine, and parapodia.

If time permits:

Examine the demonstration slide of a Lumbricus nephridium. Identify the metanephrostome and nephridiopore (refer to Brusca fig. 8, p.393). Lumbricus has the metanephridia (= open at both ends) type of nephridia.

Examine the demonstration slide of a Lumbricus nerve cord. Identify the nerve cord, medial giant, lateral giants, nerve cell bodies, subneural vessel, plexus, lateral segmental nerve (refer to Brusca fig. 8, p.393)







Systematic Classification

Phylum Annelida

Class Polychaeta (= many setae)

Subclass Errantia ("wander")

Nereis virens

Nereis cultrifera

Glycera americana

Euglycera dibranchiata


Aphrodite aculeata

Diopatra (transitional between errant and sedentary)

Subclass Sedentaria ("sit")

Pectinaria gouldii

Hydriodes sp.

Arenicola cristata

Sabellaria sp.





Class Oligochaeta (= few setae)


Class Hirudinida leeches, (external annuli do not correspond to internal segmentation)


Phylum Echiura (= serpent like), serpent worms (refer to Brusca pp. 437-444)


Phylum Sipuncula (= little tube), peanut worms (refer to Brusca pp. 444-449)