Lesson

The majority of animals on this planet are invertebrates. Of the 34 animal phla, 32 of them belong to the invertebrates that is they are lacking a spine or vertebra. Such as runs down the back of animals like dogs, horses, snakes and humans.

These invertebrates exist in huge numbers but are often leading invisible lives. yet despite their relatively invisible lives they do play a major role in ecosystem maintenance. If they were absent life would not be possible on this planet. To appreciate our own lives and our role in this world we need to fully appreciate something of the diversity of this group and the role it plays in maintaining our environment.

As a whole they are a fascinating group of animals, many being very beautiful and of great scientific interest. They assist us in helping us gain important insights into how the planet functions. In our own provincial economy we cannot understate the importance of the shrimp and crab fisheries. They provide food, both directly and indirectly and clean up the mess we make. They aid in removing both air and water pollution.

Sponges: The Phylum Porifera

The Phylum Porifera ("containing pores") contains about 5,000 species of sponges. These irregularly shaped animals lack tissues, and are usually interpreted as being at the cellular level of evolution. Cells from fragmented sponges can regenerate the sponge organism. This is not possible with animals that have tissues. They are aquatic, for the most part marine, and come in a great diversity of size, shape, and color.

Modern sponges have changed very little from their Cambrian ancestors. They may have evolved from a colonial protozoan. As previously stated there are no true tissues in sponges: simply specialized cell layers. Epidermal cells in sponges line the outer surface, Collar cells the inner cavity. The cilia of the collar cells produce water currents that flow through pores in sponge wall into a central cavity and out through an osculum, which is the upper opening. A 10 cm tall sponge will filter as much as 100 liters of water a day. Amoeboid cells occupy the "inner" layer, along with hardened structures known as spicules.

Sponges are filter feeders. That is they feed by drawing water into the body through a network of pores and passing it out through the large opening (osculum) at one end of the body.

They are able to reproduce both asexually (through budding or fragmentation) or sexually. These animals produce eggs and sperm that are released into the central cavity. From there the zygote will develop into a ciliated larva. The larval stage is can move while the adult is stationary.

Tissues: Jellyfish, Corals and Sea Anemones

The Phylum Cnidaria are characterized by adult bodies that are radially symmetrical. They are aquatic, mostly marine. They possess stinging cells called cnidocytes, that eject a barbed thread and sometimes a toxin. Only the cnidaria have, this specialized cell that contains a nematocyst, a fluid-filled capsule which contains a long, spirally coiled hollow thread. When the trigger of the cnidocyte is touched, the nematocyst is discharged. Some threads merely trap a prey or predator, while others have spines that penetrate and inject paralyzing toxins.

Two body forms are evident:

  1. a mobile medusa and a
  2. sessile (not mobile) polyp.

Both forms have tentacles arranged around an opening into the sac-like body. The inner tissue layer secretes digestive juices into the gastrovascular cavity, which digests food and circulates nutrients.

The typical life cycle involves both sexual and asexual reproduction. A bilaterally symmetrical larva known as a planula, will develop from a zygote. The planula will settle into an appropriate location and grow into the adult polyp. In turn it will reproduce asexually to form medusae. Each medusa will develop gonads and produce gametes.

Cnidarian Body Forms

Cnidarian Life Cycle

Bilateral Symmetry and Cephalization: Phylum Platyhelminthes

The Phylum Platyhelminthes contains the flatworms subdivided into three classes: two of which are parasitic and one is free-living. The planaria and its relatives are freshwater animals placed in the Class Turbellaria. Flukes are external or internal parasites belonging to the Class Trematoda. Tapeworms are internal parasites and form the Class Cestoda. The phylum as a whole has adult bilateral symmetry and cephalization (the development of a head with sensory organs.)

Flatworms have three tissue layers: ectoderm, mesoderm and endoderm and a body plan that is acoelomate and sac-like with a single opening. The mesoderm layer gives rise to muscles and reproductive organs. Free living forms have muscles, a nerve cord, and digestive organs, but lack both the respiratory and circulatory systems common to "higher" animals. Flatworms have a branched gastrovascular cavity that is the site of extracellular digestion and in turn distributes nutrients throughout the body. Gas exchange occurs by diffusion through the skin. They have an excretory system that also functions as a fluid regulating system. Flatworms have ladder-style nervous system composed of paired ganglia that form a brain connected via nerve cells to sensory cells in the body wall.

Parasitic members of this phylum, such as flukes and tapeworms, have characteristic modifications:

  1. loss of cephalization that leads to a head bearing hooks and suckers to attach to the host
  2. enhanced development of the reproductive system with loss of other systems (what do they do but reproduce?)
  3. lack of a well-developed nervous and gastrovascular system (they have no need to move or respond and the food is available in a "pre digested format")
  4. development of a thick protective layer - a teguement that protects them from host digestive juices
Class Turbellaria

The Class Turbellaria includes freshwater planaria such as Dugesia that feed on small organisms or the remains of small creatures. The planarian head is arrow-shaped, with side extensions that are sensory organs for detection of food and the presence of other organisms. Flatworms have two light-sensitive eyespots that have pigmentation making them look cross-eyed. The presence of three muscle layers allows for varied movement. Gland cells secrete a mucous material upon which the animal slides or glides.

They can reproduce both sexually and asexually. They can split beneath the pharynx and each half will grow into a whole animal by the process of regeneration. Planaria are hermaphroditic, possessing both male and female sex organs, and can cross-fertilize each other. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks.

Class Trematoda

The Class Trematoda includes the flukes. They are named after the organs they inhabit, such as blood, liver, and lung flukes. The bodies tend to be oval and elongate. They lack a definite head but have an oral sucker surrounded by sensory papillae. Flukes have reduced digestive, nervous, and excretory systems. Reproductive systems are well developed and usually hermaphroditic.

Class Cestoda

The Class Cestoda consists of the tapeworms. The tapeworm scolex (head) has hooks and suckers that allow the organism to attach to the host's intestinal wall. Behind the head is a short neck and then a long string of proglottids. Each proglottid segment contains a full set of both male and female sex organs and very little other structure. Since the animal does produce wastes, it retains its excretory canals, but no digestive system is needed. As they do not move and exist in a fairly stable environment, tapeworms have only rudiments of nerves. Following fertilization, proglottids become a bag of eggs that when mature, breaks off and passes out with the feces. If the eggs of tapeworms are ingested by other organisms, the larvae become encysted in the muscle of the hosts. If humans eat the meat of infected animals and fail to cook it properly, they too become infected. For this reason it is always recommended that pork be cooked thoroughly.

The Phylum Nemertea: Ribbon Worms

The Phylum Nemertea include the marine ribbon worms. Ribbon worms have a distinctive reversible proboscis that is stored in a rhynchocoel. When the walls of this cavity are contracted, the proboscis is extended out of the body. It is used primarily for prey capture, and for defense, locomotion, and burrowing.

Rudman, W.B., 2002 (July 6) Bootlace and ribbon worms - Nemertea. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet.cfm?base=nemertea

The Phylum Rotifera

Rotifers belong to the phylum Rotifera. They have the typical "tube within a tube" body arrangement with the digestive tract being the inner tube and the rest of the animal the outer tube. They are microscopic and abundant in freshwater systems. A crown of cilia (corona) forms a rotating wheel that serves for both locomotion and to deliver food to the mouth.

The Tube-within-a-tube Body Plan: Phylum Nematoda

The Phylum Nematoda consists of the roundworms. Most are free-living, although some are parasitic (pinworms are thought to infect 30% of all North American children). Adults have a pseudocoelom (tube-within-a-tube). This is a closed fluid-filled space that acts as a hydrostatic skeleton, aids circulation and nutrient distribution. They lack a circulatory system, but do have a well developed digestive system.

As an example of a disease that is caused by members of the phylum we can refer to the disease Elephantiasis. This disease is associated with tropical Africa, and is also caused by a worm that uses mosquitoes as secondary hosts. Adult worms reside in and block lymphatic vessels. As a result there is huge swelling of limbs where the infection has occurred. It is treatable in its early stages but not after scar tissue has blocked lymphatic vessels.

Coelomates: Animals with Internal Body Cavities

Coelomates have internal body cavities, or coeloms. We are coelomates, we have an abdominal cavity (the digestive organs and some of the excretory and reproductive organs are held there) and a thoracic cavity (the heart and lungs are found here). Coelomates also develop a variety of internal and external skeletons. These skeletons offer several advantages such as:

  1. The mineral shell that allowed the animal to use the shell as a mineral reserve.
  2. Protection from drying out during low tides for those animals that live in the intertidal zone.
  3. Predator Protection.
  4. A site for muscle attachments. This offers different means of locomotion and increased strength.

Phylum Mollusca: Clams, Scallops, and Squids

The phylum Mollusca contains a variety of body forms and lifestyles. In mollusks, the coelom is reduced and limited to the region around the heart. All mollusks have:

  1. a visceral mass containing the internal organs.
  2. a mantle that surrounds but does not entirely cover the visceral mass. It secretes a shell and also contributes to the formation of gills or lungs.
  3. a head/foot region containing sensory organs and a muscular structure (foot) that is used for locomotion. The foot is also used for attachment to the substrate, food capture, or a combination of functions. Some are slow moving, and have no head such as the scallop, while others are active predators that have a head and sense organs (eg. Squid).

The major classes of Molluscs are the:

  1. Cephaopods eg. Squid
  2. Bilvalves eg. Scallop
  3. Gastropod eg. Snail

For a video that illustrates Bivalve Locomotion Click Here.

Phylum Annelida: Segmented Worms

The phylum Annelida contains the segmented worms (such as the earthworm). These are very common to us as we are very familiar with their use as "bait" and in composting. The development of this body type allowed for specialization, different segments can perform different tasks. Their coelom is larger so as to allow for more complex internal organs. The well-developed, fluid-filled coelom and the tough outer "skin or integument act as a hydrostatic skeleton. There are marine, freshwater, and terrestrial species usually divided into three classes.

Their circulatory system is a closed one. There are blood vessels running the length of the body and extending into every segment. This more efficient means of material transport (as opposed to an open system) has allowed for a more efficient functioning. The nervous system consists of a brain connected to a ventral solid nerve cord, with a ganglion in each segment. They have a complete digestive system that include a pharynx, stomach, intestine, and accessory glands. Excretory nephridia in each segment collect waste material from the coelom and get rid of it by passing through the body wall by diffusion.

Anatomy of an earthworm. Lower (or right) image is a cross section through the earthworm Body. Note the presence of a coelom.

Classification of the Annelida
The Class Polychaeta

Most polychaetes are marine and possess parapodia ("feet") and setae ("bristles"). Parapodia are paddle like appendages used in swimming that also serve as respiratory organs. Setae are bristles, attached to parapodia, that help anchor them to the surface and also help them move. Some are active predators. Many have well-developed head regions, with well-developed jaws, eyes, and other sense organs. Some do not move and are filter feeders, they possess tentacles with cilia to create water currents and to select food particles. Only during breeding do they have reproductive organs.

The Class Oligochaeta

The class Oligochaeta are much more familiar to us as they include the earthworms. They have few setae, but those that they do have protrude in clusters directly from their body. They have poorly developed heads or parapodia. They move by coordinated movement of the body muscles with the assistance of the setae. When longitudinal muscles contract, segments bulge and setae protrude and anchor into the soil. Circular muscles contract, causing the worm to lengthen, setae are withdrawn and the segment moves forward.

They tend to live in moist soil where a moist body wall helps gas exchange. They are scavengers that extract organic remains from the soil they eat. A muscular pharynx draws food into the mouth. Ingested food is stored in a crop and ground up in a muscular gizzard. External segments correspond to internal septa (walls) separating each body segment.

The earthworm excretory system has coiled nephridia tubules in each segment with two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall. Between the two openings, the coiled nephridia tubule allows removal of waste materials from blood vessels.

Red blood is moved towards the front by a dorsal blood vessel and pumped by five pairs of aortic arches to a ventral vessel. They are hermaphroditic, with both testes, and ovaries. Mating involves the worms lying parallel to each other facing opposite directions and exchanging sperm. Each worm possesses a clitellum that then secretes a mucus, protecting sperm and eggs from drying out. There is no larval stage.

The Class Hirudinea

The class Hirudinea includes leeches. Most are freshwater, but a few are marine or terrestrial. Each body ring has several transverse grooves. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker. Although some are free-living predators, most are fluid feeders. Bloodsuckers keep blood from coagulating by hirudin, an anticoagulant in their saliva. Leeches were commonly used in early medicine to "bleed" the patient.

Phylum Arthropoda: Segmented Bodies with Segmented Appendages

The phylum Arthropoda contains animals that have segmented appendages on their body segments. Arthropods occupy every habitat virtually every niche, and are in many respects the most successful animal group on Earth. There are conservatively estimated to be over 1 million species of living arthropods.

The Reasons for their success include:

  1. A hard exoskeleton that functions in protection, attachment for muscles, locomotion, and water conservation.
  2. Jointed appendages that are specialized for walking, swimming, reproduction, etc. These adapatations account for much of the diversity and success of arthropods.
  3. A relatively complex nervous system with a brain connected to a ventral solid nerve cord.
  4. A unique respiratory system that employs many different types of respiratory organs.
  5. A complex, yet adaptable, life cycle. Metamorphosis is a drastic change in form and physiology that occurs as an immature stage becomes an adult. Metamorphosis contributes to the success of arthropods because the larval stage eats food and lives in environments different from the adult; reducing competition between immature and adults of a species. Reduction in competition thus allows more members of the species to exist at one time.

The arthropod body consists of three major body segments:

  1. head
  2. thorax
  3. abdomen
Phylum Echinodermata

There are about 6000 species of echinoderms; all of which are marine. The phylum Echinodermata includes the sea urchins, sea stars, sea cucumbers, and starfish. Mosthave radial symmetry, while the larvae are bilaterally symmetrical. They have an endoskeleton consisting of calcareous plates with spines. The adult body form appears to be an advantage to the mostly bottom-dwelling echinoderms, who can as a result feed in every direction. Adult echinoderms have no brain. Members of the phylum have a water vascular system that powers their multitude of tube feet.

Anatomy of an adult sea star. Lower (right) image is a cross section through an arm of the adult sea star.

Activity

  • Read Pages 182 - 189 in McGraw Hill Ryerson
  • Answer Questions 14 & 17 on Page 197 - 198 of McGraw Hill Ryerson. Submit your answers to your instructor
  • Download, Print, and Complete the Following Table - Invertebrate Worksheet. Submit it to your instructor.

Test Yourself

Click here to complete the self test for this lesson.