13.1: Lab 13 Background

Learning Objectives

• Identify invertebrates to the phylum level based on visual characteristics or a list of traits.
• Build a dichotomous key to describe differences in traits between animal phyla.

Introduction

All animals are multicellular, heterotrophic eukaryotes that lack a cell wall and are motile at some point during their life cycle. Take a moment to look over the phylogenetic tree in Figure 1 below. Identify which animals are considered invertebrates. These are the phyla we will learn about in this lab.

Figure 1. A phylogenetic tree of the kingdom Animalia showing only the ten most species-rich phyla. Together, these phyla cover the majority of living animals. From Discovery Biology, 6/e Figure 9.2.

Porifera

Animals in the phylum Porifera are commonly known as sponges. Porifera are among the first animals to have ever evolved! The cells of these organisms come together and collaborate, but are not true tissues. This means that Porifera are multicellular pore-bearing animals with no organs. It may be difficult to imagine, however, sponges only consist of individual cells that come together to make the whole work. An analogy is to think of a classroom with students who come together to be a whole class, yet remain as individuals. When sponges die, we can use their remnants for bathing, known as a natural sponge which is similar to a loofah (though, a traditional loofah comes from a fruit!). These animals may appear to have radial symmetry, however, animals with true radial symmetry also have true tissues. Therefore, sponges are considered to lack any symmetry. Sponges can be very large and can regenerate portions that get damaged or break off.

Cnidaria

Animals in the phylum Cnidaria have simple tissues and true radial symmetry, though they lack true organ systems. Examples include jellyfish (also called jellies because they’re not fish), sea anemones, and coral. Cnidarians tend to have two body plans: polyps are tube-shaped with the tentacles facing upward (like coral and anemones), while medusae (singular: medusa) are bell-shaped with the tentacles facing downward (like adult jellyfish). Some cnidarians, like jellyfish, live in both the polyp and medusa stage during different life stages. Tentacles have specialized stinging cells called cnidocytes that are used for catching prey and defense. With a simple nervous system, they can move their tentacles towards their mouth, and free-floating medusae contract simple muscular tissue for swimming. These animals can reproduce both sexually and asexually.

Mollusca

The mollusks have a relatively complex body plan with multiple types of tissues and organ systems. This is a very diverse phylum and contains the highest number of marine species of any animal phylum. Examples of mollusks include shellfish (like clams, mussels, and scallops), gastropods (like snails and slugs), and cephalopods (like octopi, squids, and cuttlefish). Mollusks have soft bodies with distinct regions, including a muscular foot and a mantle that sometimes secretes a shell. They have complex organ systems with most of their organs grouped together in the visceral mass. Cephalopods, especially octopi, are often considered the most intelligent invertebrates on Earth.

Platyhelminthes

These worms, commonly called flatworms, are the simplest of all worms because their bodies are made of simple tissue layers and do not have a distinct body cavity. As we see in Figure 1, flatworms, along with all other animals except poriferans and cnidarians, are bilaterally symmetrical. They were also among the first animals to evolve organ systems, including simple digestive and nervous systems. They lack a respiratory system, but their thin bodies allow gas exchange through diffusion, which makes them dependent on an aqueous environment. Flatworms may be parasites or free-living in fresh water, salt water, or saturated soils.

Annelida

The most complex worms are commonly called segmented worms, including earthworms and leeches. These worms have a true body cavity (coelom), advanced digestive & nervous systems, and distinct body segmentation. The cuticle that covers their soft cylindrical body allows them to absorb oxygen. Different species thrive in soil or in water. Some segmented worms have paired extensions that protrude from their bodies, called parapodia, for locomotion.

Nematoda

These worms are commonly called roundworms and may exist in aquatic or terrestrial environments. Their bodies are more complex than flatworms because of a distinct mouth, anus, and more complex digestive system surrounded by a semi-filled cavity called a pseudocoelom. Roundworms have a tough outer layer, soft cylindrical bodies, and no segmentation. Many species of Nematoda parasitize humans, including the hookworm and pinworm.

Arthropods

Arthropods are the most abundant and diverse group of eukaryotes, let alone invertebrates. Estimates vary, but of the 2.1 million eukaryotic species described, arthropods make up over 1.2 million of them. Of the arthropods, insects are the most diverse by far, consisting of over 1 million species! Insects were also among the earliest terrestrial animals. Other arthropods include crustaceans (like crabs, lobsters, and barnacles), arachnids (like spiders and scorpions), and myriapods (like centipedes and millipedes). Some species are aquatic and others terrestrial; some are even parasitic, like mites and marine isopods. The tissues and organ systems of these animals are well developed. Their sensory organs allow them to react quickly when hunting or escaping. All arthropods have a hard outer body layer called the exoskeleton, which is composed of chitin and provides support to pairs of jointed limbs. In fact, the word arthropod means “jointed foot”. The body is clearly divided into three sections: head, thorax, and abdomen. Insects’ exoskeleton and wings are two of the reasons they have been so successful for hundreds of millions of years.

Echinodermata

Echinoderms, including sea stars (also called starfish, but they’re not actually fish), sea urchins, and sand dollars, have a hard outer body made from calcium carbonate, as opposed to the chitin that makes up arthropod exoskeletons. In fact, the word echinoderm means “spiny skin”. However, there are some soft bodied echinoderms such as sea cucumbers. All echinoderms live in the ocean. Their embryonic development sets them apart from other invertebrates; as deuterostomes, the anus develops first before the mouth in the embryo. They have a true coelom, though their body shape may be round or star-shaped because of multiple appendages anchored at a central body. As larvae, echinoderms are bilaterally symmetrical, but they are radially symmetrical as adults. Many echinoderms move using a unique water vascular system and tube feet. Echinodermata is the sister phylum to chordates, which we’ll learn about next week.