Chapter 26 Notes - Animals: The Invertebrates

A. What characteristics do all animals share?

B. Key aspects of anatomy for purposes of classification –

Below is a listing of the major invertebrate phyla. If you can remember a few key characteristics for each group, you should be fine on the AP test. From reviewing past tests, it seems the focus is one how they are classified (e.g., what makes a given animal an arthropod or cnidarian).

A.Porifera (the sponges)

Most people don’t recognize that natural sponges are actually the remnants of an aquatic animal. In terms of anatomy, the sponges are the simplest animals known. They display no symmetry and have no specialized gut, body cavity or body segments. They largely consist of protein and spicules of glass-like silica. They are able to pull water into a central cavity by motion of flagella and captured food is absorbed and digested by amoeba-like cells within the sponge, but there is no distinct digestive organ. They are able to reproduce sexually and, as larvae, they are motile (mobile) animals.

B. Cnidarians

The cnidarians are tentacled aquatic animals with radial symmetry. Examples are anemones, sea jellies and corals. In the cnidarians, tissues are evident (tissue is made up of similar cells which perform a similar function). In particular, there are three notable aspects of cnidarian anatomy: 1) nematocysts, complicated triggers with nerves that shoot out a barbed stinger in reaction to a stimuli; 2) saclike gut, with one opening; and, 3) epithelium, tissue made of cells with one side facing an exposed environment (gut lining, skin, etc). Cnidarians do not display a true body cavity. They maintain their shape by moving water within a hydrostatic skeleton, in which contractile cells move water from one cell to another.

C. Flatworms and Ribbon Worms (Acoelomates)

In the flatworms, organ systems are observed for the first time. This group includes animals with a head (cephalization), bilateral symmetry, saclike gut and a pseudocoel. Within the pseudocoel a branching digestive system and developed sex organs are found. Additionally, a nervous system is found within these animals. Representatives of this group are commonly found in Splitrock Creek (planarians) and were at one point added to diet pills (tapeworms). The ribbonworms are a bit more developed, including complete gut, a circulatory system and separate sexes.

D.Roundworms

Roundworms are similar to flatworms except they display a pseudocoel and a complete digestive system. They also have a cuticle (in animals, this a protective coat, not the waxy coat found in plants). Roundworms are responsible for some significant health problems in humans, including trichinosis (from eating undercooked pork) and the form of elephantiasis scene on page 435 of the text.

E. Rotifers

These organisms are similar to roundworms in diversity of body plans, but in a very different form. They have a ring of cilia around the top of the head and pack a great number of organs into their small, nearly translucent bodies.

F. Mollusks

The mollusks are the first animals with a true coelom. This very diverse group includes snails, clams, squid, cuttlefish and octopuses. The name (think mollify) refers to their soft body. This body includes most organs commonly associated with animals: heart, stomach, mouth (often a series of teeth for grinding or scraping, known as a radula), anus, and gills (thin tissue for gas exchange). All mollusks have a visercal mass (consisting of organs), a mantle (tissue around the visceral mass consisting of gills and the cells capable of depositing the calcium carbonate shell) and often a ‘foot’ used for movement.

The mollusks are such a diverse group that we will note three divisions (classes). A) Gastropods are a class containing snails and conch shells. They usually have a shell protecting their soft bodies and they move by use of a large "foot," which is a meat commonly eaten in coastal areas. This foot is actually their body that can extend out of the shell and move against the ground, thus known as a pseudopodia (false foot).

B) Bivalves, which includes clams and scallops. The bivalves are so named as they have paired valves, one for intake and one for output of water. Using the valves, they can move very quickly by means of "jet propulsion."

C) Cephalodopods have lost their external shell and are now recognized as octopus, squid and cuttlefish. They do have an internal shell, but their main adaptations are a large head (the class name refers to their large head) and rapid movement. Represenatives of the cephalodpods are often considered the most intelligent of the invertebrates.

G. Annelids

Common examples of annelids include worms and leeches. These organisms display segmentation, allowing for specialization of body parts. Annelids have permeable skin, a complete gut and true coelom, and a developed organ system. Most worms, for example, include a basic brain (at this level, it is a congregation of nerves useful for coordinating some nerve activity, not developed enough for any serious ‘thinking’ – a lot like the brains of many high school freshmen, although they aren’t as coordinated as many worms, possibly due to those stupid baggy pants so many of them wear). Worms often have a series of aortic arches (hearts) that move blood through a closed circulatory system (contain vessels allowing the flow of blood to specific locations within the body). Also, a series of nephridia move liquid from one segment to another, an important function because most annelids use hydrostatic pressure to move their bodies as they lack a skeleton against which muscles can pull.

H. Arthropods

I have heard estimates that each human being on Earth is outnumbered 100 million to one by the insects alone. While we often crushed, swat, and step on them, insects and the other arthropods deserve a great deal of credit for the things they do for us.

There are three living lines (classes) of arthropods: chelicerae (spiders and ticks), crustaceans (shrimp, crabs, barnacles, lobster), and uniramians (insects and millipedes). As your books describes this large group of animals, they display six unique adaptations:

1)hardened exoskeleton,

2) jointed appendages,

3) fused/modified segments,

4) unique respiratory structures,

5) developed nervous/sensory systems, and

6) a division of labor in life cycle.

Below is an outline of the adaptations seen in each class:

Chelicerates – This group includes spiders, boatmen, chiggers and ticks, horseshoe crabs, and scorpions. As a group, they are diverse, making it difficult to classify them by a few shared characteristics. In general, they have twelve legs (often eight are used for walking), two body segments, open circulatory systems and developed book lungs. Section 26.20 highlights some diseases transferred via chelicerae (for example, the bacterial infection Lyme’s disease).

Crustaceans – Think of a lobster as a model crustacean. It has a very hard exoskeleton, jointed legs (walks on five pairs), modified front legs for grasping and handling food, and modified segments to assist is breathing and motion. Also, most crustaceans display antennae and eyes as part of a complex sensory nervous system.

Uniramians – Insects and their kin are remarkably diverse. The insects have three body parts (head, thorax and abdomen) and extensive cephalization. They usually walk with three legs and they often display wings. In their lifecycle, they will go through complete (egg, larvae, pupa, adult) or incomplete (egg, nymph, adult) metamorphosis. Insects usually breath through openings in the body (trachea) that allow oxygen to diffuse directly into body tissue, they excrete nitrogenous wastes (from protein synthesis) in the form of uric acid crystals (saving water) and they have a series of Malpighian tubules that allow them to channel wastes through the gut.

I.Echinoderms (spiny skin)

The echinoderms include sea stars, sea urchins, and sea apples/cucumbers. The defining characteristic of the group is calcified body armor (evident in sea urchins, less obvious in sea stars). These organisms often display radial symmetry, an odd deviation from the evolutionary line. In other features, they have developed features: full gut, coelom, nerves, a water-vascular system that helps them move, and (often) bilateral symmetry of larvae. Why do they display basic features such as radial symmetry, lack of cephalization, and no central brain? One hypothesis is they have lost these features to allow each "arm" or extension to function effectively by itself. Internally, most are rather complex. They often display a water-vascular system unique to the echinoderms, which allows them to move by filling tube feet with water (like sea star in the tank).