Exercise 19 - Protostomes I: Annelids and Molluscs

STUDENT OBJECTIVE

Students dissect an earthworm and a clam. A demonstration dissection of a squid is available in the lab. Diversity in the molluscs is illustrated. Segmentation and tube-within-a-tube body plans are illustrated.

EQUIPMENT AMOUNT (Class of 24 with 8 groups)
Compound microscope 1/student
Dissecting microscope 1/student


MATERIALS

Living specimens
  Earthworms (night crawlers) 1/student
Preserved specimens
  Polychaete worms, whole and dissected Demonstration
  Leeches, whole Demonstration
  Chiton, whole and dissected Demonstration
  Snail, whole Demonstration
  Squid, whole and dissected Demonstration
  Scaphopod, whole and dissected Demonstration
  Clam, whole 1/student
Prepared slides (sharing possible to reduce costs)
  Earthworm, cross section (CBS#Z1250)* 1/student
  Clam, cross section (CBS#Z1490)* 1/student
Beaker 2/lab
Dissection pan 1/student
Dissecting instruments: scissors, forceps, blunt probe,
razor blade or scalpel, pins 1/student

SOLUTIONS

10% ethanol

PREPARATION

About Three Weeks before Lab

A biological supply house or local source of large earthworms should be located and an order placed for the week of lab. Live worms can be kept in a refrigerator. Preserved specimens and prepared slides should be ordered as needed to arrive one week before lab.

One Week before Lab

  1. Inspect arriving orders for suitability.

  2. Clean and check dissecting instruments.

  3. Ethanol preparation:
    10% ethanol 50 ml 95% ethanol/425 ml water

Day before Lab

Set up specimens demonstrating diversity of annelids and molluscs. Prepare squid dissection and cover with wet cloth and plastic wrap.

CLASSROOM SUGGESTIONS

A slide show on diversity can be developed with Kodachrome slides from various sources and run continuously in a hallway showcase using a rear projection, automatic projector (Caramate). If the library has a media center, a slide-tape show on diversity can be developed and made available there. The biology videodisc available from Wm. C. Brown Publishers contains several images that illustrate diversity and could be interfaced with a computer in an instruction/quiz format.

ANSWERS TO CRITICAL THINKING QUESTIONS

  1. Earthworms exchange gases across their moist body surfaces with the air. In heavy rains, their burrows become filled with water. Water has a lower concentration of dissolved oxygen than air. Selection has favored those worms whose behavior brings them to the surface as oxygen levels in the blood drop.

  2. Annelids and molluscs have several well developed organ systems and are organized on a bilaterally tube within a tube body plan. Their behavioral capabilities are greater than those of nematodes and cnidarians. While nematodes also have a bilateral tube-within-a-tube body plan, their organ systems are not as well developed. The cnidarians have an incomplete digestive system and lack well defined organs beyond the reproductive system. They are radially symmetrical and not as behaviorally diverse as the others.

SUPPLEMENTAL MATERIALS

Bio Sci II, videodisc--contains pictures of these animals. Dubuque, IA: Wm. C. Brown Publishers. See appendix.

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