# 1.2: Invertebrates Diversity Teacher's Preparation Notes

## Teaching Points

• Introduce students to some of the variety of animal form and function, including varied methods of locomotion
• Relate form to function, including the advantages of bilateral symmetry and cephalization
• Distinction between similarities due to shared evolutionary history (animals in the same phylum) and similarities related to having a similar lifestyle (burrowing animals)

## Equipment and Supplies

(For 6-10 students)

• Containers to display the invertebrates in:
• 2-3 medium-sized containers such as Gladware’s Soup & Salad size (3 cups)
• 1 large-sized container as large or larger than Ziploc’s large Rectangular Container (9.5 cups) to allow room for crayfish movement
• 1 gallon-sized plastic bag
• 1 plastic tray or plate
• Magnifying glasses or hand lenses (4-6) and, if available, dissecting microscopes
• Rulers (1-2)
• Dechlorinated tap water
• Gloves for each student (optional; if you do not have students use gloves, make sure they wash their hands after handling the animals.)

# Purchase from the Local Pet Store

(e.g. World Wide Aquarium & Pets, 7043 Ridge Ave, Philadelphia, PA 19128, Roxborough) (approximate prices)

 Item Price Crayfish (1-2) $1.79 for 2 Crickets (5)$0.10-0.20 each Mealworms (5) $0.10-0.20 each Earthworms (5)$2.59 for 24

This set of animals is enough for approximately 10 students. You can reuse the animals all day but, depending on how many classes you teach, you may want to buy replicate sets to ensure freshness and mobility for each class.

## Instructions:

For every 6-10 students, set up 2 stations:

1. Place 5 mealworms in a medium container. Place 5 earthworms on a damp paper towel on the tray or plate. Have 1 or 2 rulers available at this station and, if available, a dissecting microscope. When the students are not actively observing the earthworms it is very important to keep them moist by covering them with a wet paper towel.

2. Place 1-2 crayfish in a large uncovered container filled with dechlorinated tap water. Keep the crayfish separated overnight so they will not injure each other. Set out five crickets in 1-2 containers. Crickets are best viewed in a plastic bag that is expanded to full volume (either the bag they come in from the pet store or a plastic storage bag).  However, you cannot keep them in a sealed bag overnight and must store them in a different covered container with a screen on top. Alternatively, you can display the crickets in a plastic container. Poke air holes in the bottom of 1-2 medium containers, place the crickets in the containers, put on the lid, and place it upside down. Be careful that the crickets don’t jump away during transfer.

Have several hand lens or magnifying glasses available at each station.

## Teaching Suggestions and Biology Background

Before beginning this activity, students should be familiar with basic concepts such as phylum, class, bilateral, and radial symmetry. If you have a 40-50 minute lab period, you may want to plan to discuss the Follow-Up Questions in the next class period, so students will have plenty of time to observe the animals.

You may want to use the introductory section (page 1 of the student handout) as part of a lecture/discussion for the class period before the lab activity. This could include an introduction to the annelid, arthropod, chordate phyla, and major subphyla like the Crustacea as well as examples of other animals included in these phyla, subphyla, and classes.

## Characteristics of Observed Invertebrates

 Name Earthworm Mealworm Cricket Crayfish External Anatomy segmentation visible, clitellum* legs, small antennae, very small eyes, segmentation visible legs, wings, eyes, antennae, segmentation visible in the abdomen legs, claws (chelipeds), eyes, antennae, segmentation especially visible in the abdomen How does the Animal Move? hydrostatic skeleton; alternating shortening and elongation of different segments walks on legs on the front of the body legs and wings (walking, jumping and flying) swims backward with tail, walks forward on legs

*The clitellum secretes material that makes the cocoon which surrounds the eggs and fertilizing sperm when they are released by the hermaphroditic earthworm after copulation.

The following figure from Biology 6th Edition, by Campbell and Reece, may be helpful for clarifying student observations concerning earthworm locomotion.

You can use the comparison between earthworms and mealworms to contrast phylogenetic categories with non-phylogenetic common usage categories such as worms (animals with one dimension much longer than the other two). A phylogenetic category, such as a phylum, groups animals that share a common evolutionary ancestor and therefore share similarities in their fundamental biology. Evolutionary relatedness is judged based on characteristics that often are not obvious from the outside, so animals that have very different external appearance may be grouped in the same phylum (including larval forms such as mealworms and caterpillars which are grouped with all other insects in the Arthropod phylum). In contrast, animals that look similar but have very different internal anatomy may be grouped in different phyla (e.g. flatworms, roundworms, and segmented worms).

All of the animals in this activity are bilaterally symmetric. Bilaterally symmetric animals typically have a concentration of sensory organs at the head end of the animal, which allows the animal to gather information about the environment it is moving toward. The concentration of sensory organs and much of the nervous system at the head end is called cephalization. In contrast, radial symmetry is observed in organisms like jellyfish or hydra which drift slowly through the environment or are sessile; radial symmetry is associated with sensory organs distributed around the circumference which receive sensory information from all directions.

Follow-up Questions 3-6 guide students in thinking about similarities due to shared evolutionary history vs. similarities due to adaptations to similar environments. If your students have studied homology and analogy, you may want to link this discussion to homology (similarities due to shared evolutionary ancestors) and analogy (similar form for similar function due to convergent evolution). With regard to Follow-up Questions 4 and 5, both the worm shape and the absence of substantial eyes are related to the burrowing lifestyle. Earthworms live underground and consume decaying organic material while mealworms live surrounded by what they eat (e.g. grain or grain products).

You may want to contrast the type of development in mealworms/Darkling beetles (complete metamorphosis) vs. in crickets (incomplete metamorphosis). Complete metamorphosis is observed in insects where the larval stages look completely different from the adult (e.g. mealworms or caterpillars) and the transformation from the largest larva to adult occurs in a pupa; the larval stages are specialized for eating and growing and the adult stage is specialized for dispersal and reproduction. Incomplete metamorphosis is observed in insects like crickets where the young resemble the adults, although they lack wings; each molt produces a larger insect with more nearly adult body proportions, and the final molt produces an insect with wings and mature reproductive organs.

For additional information on the anatomy, biology, and care of these animals see the following websites:

Earthworm (e.g. Lumbricus terrestris)

Mealworm, larvae of Tenebrio molliter

Cricket, Acheta domestica

Crayfish (e.g. Procambarus or Orconectes species)

Unless you are very familiar with these organisms, we recommend that you have access to these sources or an invertebrate zoology textbook during the activity to help you answer student questions. Also, the figures from these sources can help students to interpret observed characteristics.

[1]              These teacher preparation notes and the related student handout are available with a CC-BY-NC 4.0 license at http://serendip.brynmawr.edu/sci_edu/waldron.

[2]              You can dechlorinate tap water by leaving it out in the open containers overnight or by adding a commercial dechlorinator used for aquarium water.  Alternatively, you can ask for extra from the pet store.