Chapter 12 The Invertebrates

Transcription

1 Chapter 12 The Invertebrates zoology - the study of animals General Characteristics of Animals multicellular eukaryotic heterotrophic - must obtain and ingest organic food. Classification by notochord The animal kingdom is divided into two broad groups: vertebrates - animals that posses a notochord (spinal chord) at sometime in their life. invertebrates - animals that lack a backbone throughout their life. Embryological Development in Animals All life begins as a single cell (fertilized egg) and goes through certain stages in it s development to a multicellular organism: Zygote - fertilized egg multiple divisions Blastula - a hollow ball of cells formation of germ layers: 1

2 ectoderm - outer layer. Will become skin, nervous system, nails, hair, scales endoderm - inner layer. Will become the lining of the gut mesoderm - middle layer. Will become internal organs and muscles gastrulation - an infolding on one side Gastrula The degree of complexity between animals depends on how much the germ layers differentiate (become different types of cells). For example: jellyfish and sponges have no mesoderm because they do not develop internal organs or muscles. Water circulates freely throughout them delivering nutrients directly to each cell and carrying away waste at the same time. There is no need for a transport system. complex animals have internal organs and transport systems in order to get past the lack of water environment. Classification by body cavity (coelom) coelom a lined body cavity which houses the internal organs 2

3 acoelomate - no coelome flatworms (Platyhelminthes) pseudocoelomate - false (fluid-filled) coelom round worms (Nematoda) coelomate true body cavity lined with peritoneum. vertebrates Classification by symmetry - (the general body plan). Can you get a mirrored image if you cut this thing down the middle? asymmetry - having no orientation of body plan - no front/back/top/bottom. E.g. Sponges (Porifera) radial - having symmetry about a central axis. E.g. Jellyfish, hydra (Cnidaria) and starfish (Echinodermata) bilateral - two sides. Symmetry between a single plane. This leads to directionality. 3

4 Classification by cephalization - the degree of brain development. (The speed and direction at which an animal moves determines degree and position of nerve development.) WHY? 4

5 Copy Table 12.1 Summary of Majour Phyla into Notes Phylum Porifera (Think: no tissues!) organization Sponges are considered the simplest of multicellular animals because although they have specialized cells, the cells are not organized into specialized tissues (remember the hierarchy ) feeding figure 12.7, 12.8 reproduction Sponges are hermaphroditic because they produce both egg and sperm, but they do not self-fertilize because the gametes are produced at different times. Zygote is free-swimming so it can travel some distance before it settles down to start a new sponge. Because 5

6 sponges are so simple, they can also reproduce by fragmentation (when part breaks off). ecology Many sponges form symbiotic relationships with photosynthetic bacteria and protists. Some even form associations with small shrimp. Phylum Cnidaria (Think tissues and radial symmetry!) organization Cnidarians have true tissues (with a middle layer of jelly-like matter called mesoglea) and are radially symmetric. Most of their 10,000 species live in the oceans (marine) while only about 50 species are freshwater dwellers. They have a primitive nerve net but no true central nervous system (i.e. no cephalization). Have two body forms: medusa and polyp. Some cnidarians live as medusoids (jellyfish) while others live as polyps (hydra). Still others show alternation of generations between the two forms (Obelia) (more on that later) feeding Use poisonous harpoons called nematocysts which are stored like coiled springs inside specialised cells called cnidocytes (hence the phylum name). When prey swim by, they inadvertently trigger a stinger missile - ahh! Lunch! Food enters a blind digestive sac, the contents digested and waste expelled through same opening. Can eat larger prey than sponges. WHY? reproduction 6

7 medusoid forms - all produce sexually. Pseudo-gonads (false reproductive organs) arise from epidermis, produce egg and sperm, which is released into the water for fertilization, zygote settles and starts again. Polyp forms - generally produce asexually by budding under good environmental conditions and sexually if conditions are not so great. In this case the zygote will become a dormant cyst which will hatch once conditions improve. Obelia - Alternation of Generations figure specialization In Obelia there is some degree of tissue specialization. Note some polyps are reproductive, some are for feeding. This shows a division of labour between tissues. 7

8 ecology There are many symbiotic relationships between fish, arthropods, turtles and other small animals. Often the corals and jellyfish provide protection while the other organisms provide a cleaning service. Read p Ecology and Deep Sea Water Vents. Phylum Platyhelminthes (Think: bilateral symmetry and a primitive brain!) What sets Planaria and other flatworms above the previous organisms are the following true organs primitive organ systems some cephalization two nerve cords ending in a concentration of nerves eyespots ability to learn, co-ordination of movement. Negative phototaxic, positive chemotaxic. blind digestive tract free-living and *parasitic forms rudimentary excretory system reproduction by sexual means (copulation) and asexual (fragmentation) Adaptations to a parasitic lifestyle. Limited size. Why? To keep host from being used to death. Lack of sensory structures. Why? 8

9 Lack of complex digestive structures. Why? Highly complex reproductive system. Why? Development of a protective, modified epidermis. Why? Sometimes a cuticle is present. Why? Aschelminthes Pseudocoelomates A large taxa of worms which are considered bridge organisms between the lower acoelomates and higher coelomates. Common characteristics include: a complete digestive tract one way digestion! WHY IS THIS IMPORTANT? a primitive brain and nerve cords running the length of the animal no respiratory or circulatory system (transport is by diffusion). They include a rather disgusting phylum of worms called nematodes many of which are parasitic on almost every life form known Phylum Nematoda Their ecological effects are far reaching and devastating as they infest crops, domesticated cattle and humans. Read p314-5 Life Cycle of Tapeworm 9

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11 Ascaris lumbricoles A large mass of Ascaris lumbricoides that was passed from the intestinal tract. The ruler at the bottom of the image is 4 cm (about 1.5 inches) in length. 11