Practical Course in Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Annelids
Class Polychaeta

Nereis.

Typical Representatives of the Class of polychaete worms (Polychaeta) are various nereids, which are abundant in marine environments (locally, for instance, in the Barents, White, Black, and Azov Seas). They play a significant role in the diet of fish.

Materials and equipment. The genus Nereis is represented in our seas by several species. For dissection, it is preferable to use larger specimens, whereas smaller worms should be selected for fixation.

Nereids are harvested using a hand net among Algae or with a dredge. External Morphology is studied using live or alcohol-preserved worms, as well as whole mounts (HEAD region, parapodia). Internal anatomy is examined through dissection and cross-sections.

The required equipment includes: a Microscope, a dissecting microscope, a hand lens, a wet mount of Nereis, fixed worms, Petri dishes, dissecting needles, forceps, and a slide mount of a parapodium.

Fig. 58. External Structure of Nereis:

A — head region; B — trunk region; C — anal region; 1 — tentacles; 2 — palps; 3 — prostomium; 4 — peristomium.

Fig. 59. Parapodium of Nereis:

1 — dorsal cirrus; 2 — notopodium (dorsal lobe); 3 — neuropodium (ventral lobe); 4 — ventral cirrus.

Fig. 60. Anterior region of the polychaete annelid Nereis:

1 — tentacles; 2 — eyes; 3 — parapodia.

Assignment. Examine and sketch the external anatomy of Nereis, including its anterior and posterior ends, as well as one of the parapodia.

Examination of a wet mount or fixed worm in a Petri dish. Examine Nereis using a hand lens or a dissecting microscope. The body of polychaetes is distinctly divided into head, trunk, and anal regions (Fig. 58). Observe the head end from the dorsal side; it consists of the head lobe and the oral segment, or prostomium (Fig. 60). The head lobe bears a pair of tentacles and a pair of long, thick palps (chemoreceptors), along with two pairs of eyes. The oral segment features four pairs of long, slender cirri on its anterior margin and the Mouth opening on the ventral side. Parapodia are situated along the sides of the trunk segments. Examine the posterior end of the body, noting the terminal anal segment equipped with anal cirri.

Examination of a microscopic slide of a parapodium. Under low magnification, examine The structure of a Nereis parapodium (Fig. 59). It consists of a basal part divided into two lobes: dorsal and ventral. Locate the dorsal and ventral cirri associated with each lobe. The cirrus Functions as an Organ of Touch. Observe the bundles of sturdy, transparent needles (setae) protruding from the tip of each branch. Both lobes also possess an additional inner thick supporting bristle (aciculum).

Parapodia are outgrowths of the Skin-Muscle sac that serve as locomotor Organs. In some polychaetes, the dorsal cirrus of the parapodium is modified into a gill. Studying the structure of polychaete parapodia provides insight into the evolutionary origins of arthropod limbs, which evolved from annelid ancestors.