Lecture 7 Notes
The Animals
| I. What makes an animal?
A. all animals are heterotrophic B. All are multicellular C. Animals do not have cell walls - all cells are bounded by plasmamembrane only D. Animals evolved from protists - most likely aquatic protists (in water) E. Of the 35 phyla in the Animal Kingdom, only 2 have become successful on land 1. Arthropods - (spiders and insects) 2. Chordates - (animals with backbones) F. General Features 1. Except for sponges, cells in animals are organized into tissues a. tissues are a group of cells that operate as a functional unit (skin, muscle etc.) b. different tissues can be grouped together into a higher order of organization, called organs 2. Most animals are diploid and reproduce sexually, but there are the few exceptions 3. Once the egg is fertilized by the sperm, the embryo develops in a highly ordered way a. a series of mitotic divisions gives rise to a structure which is a hollow ball of cells called a blastula b. after a while, the hollow ball of cells folds in at one end, and an opening forms at the other end - opening known as a blastopore c. An embryo with a blastospore is called a gastrula d. Subsequent growth produces a digestive system, and other tissues and organs G. Sponges 1. about 10,000 species in marine waters, only about 150 in freshwater 2. these are the simplest animals 3. cells not organized into tissues as we know them 4. there are no organs 5. sponges lack symmetry - all other organisms have some sort of symmetry 6. Very little coordination among cells, even though all growing as one entity 7. Represents early attempt at multicellularity 8. You can pass a sponger through a sieve, and cells will eventually re-assemble as a sponge organism all by itself. Quite amazing. 9. Body of sponge made of several types of cells a. amoeboid cells - can move about in body of sponge i. similar to amoeba in their shape and movements b. choanocytes (collar cells) - have flagella and draw food into sponge i. these cells probably evolved from flagellated protists c. sponge body also interfused with silica crystals (probably a herbivory deterrent) d. epithelial cells - form outer surface of sponge H. All other animals 1. have symmetry 2. are composed of 3 layers of tissues a. ectoderm - outer most layer (skin, coat, nervous system) b. mesoderm - middle layer of tissues - (muscles and skeleton) c. endoderm - inner most layer (lining of intestines) I. Only two phyla are radially symmetric (circular in profile) 1. Cnidarians - jellyfish, hydra, sea anemone, corals 2. Ctenophora - comb jellies 3. Radial symmetry means that if you cut through the organism, the two halves are identical, and it doesn't matter what radius you pick to make the cut (Pizzas are radially symmetrical, and you can cut them in half anywhere along their radius) 4. All other phyla are bilaterally symmetric - that is, they have a left and right side that are mostly identical. 5. All cnidarians are carnivores - eat other animals (no vegetarians here!) 6. Cnidarians come in two forms a. polyps - pipe-shaped animals, usually attached to rocks or other hard surfaces. Mouth faces up to get food. b. medusae - free-floating and umbrella-shaped. Have hanging tentacles and mouth faces down. c. some cnidarians alternate between being polyp-like and medusae-like at different times in their life cycle. Often medusae as a juvenile, polyp as an adult (corals) d. some medusae (jellyfish) have hanging tentacles with stinging cells. Such cells called nematocysts. Touching causes rush inward of water, and expulsion of a barb or stinger that injects poison. Some toxins are the most deadly of any toxins known. e. major evolutionary advancement - digestion in a gut, extracellularly, or, outside of single cells. f. Cnidarians have a gut with only one opening - food enters and leaves through same opening. g. Cnidarians have incomplete digestion i. food broken down into small particles in gut ii. cells lining gut then absorb small particles and complete digestion iii. absorbtion of food into cells by process known as endocytosis J. Bilaterally Symmetric Animals 1. All of these animals have a front and back end 2. Three basic body plans a. Acoelomate - most primitive i. no body cavity - just a central digestive system b. Pseudocoelomate i. body cavity develops between mesoderm and endoderm, not within mesoderm c. Coelomate - most advanced (we are coelomates) i. organs are suspended inbetween layers of the mesoderm 3. Examples of each type of body plan a. Acoelomate i. Solid worms (flatworms) 1. many are parasitic and simple in organization 2. can be small (1 mm) or long (several meters) 3. no circulatory system, gut usually has just one opening b. Pseudocoelomate i. Nematodes - about 12,000 species known, many more remain to be discovered - some estimate there could be 500,000 species!! ii. cylindrical, unsegmented worms iii. usually very small iv. digestive system more advanced - food passes through and digestive system has an opening to take in food, and one to pass out wastes v. many are pests of crops - attack roots especially vi. others are parasites on animals (roundworms, hook worms) c. Coelomate i. Humans ii. Evolution of body cavity 1. If you have a solid body, with no circulation system, you're limited in what you can do physically - muscles would always be pressing or deforming organs every time you moved 2. An internal body cavity eliminates these problems a. internal organs suspended in a fluid-filled space - avoids deformations, and it acts as a shock absorber b. further, fluid surrounding organs can transport gases and hormones, and nutrients, etc. c. Isolation of gut allows more control over digestion and greater efficiency d. coiling of digestive system allows long length in small space - now animal can more efficiently extract nutrients from food and it can store food so it doesn't have to eat constantly iii. There are two kinds of coelomates 1. In first group, mouth develops near blastospore a. includes mollusks, annelids (segmented worms) and arthropods (lobsters, crabs, insects and spiders) b. this pattern of development widespread, and may have been the initial pattern for all animals - is the more primitive pattern 2. Second group - includes echinoderms (starfish) and chordates (animals with backbones - us!) a. here, anus (butt) forms from or near blastospore, and mouth forms from another portion of blastula b. hence, starfish are more closely related to us than insects or crabs iv. Mollusks 1. Includes snails, clams, scallops, oysters, cuttelfish, octopus, slugs, etc. 2. Three classes are recognized a. Gastropoda - snails, limpets, and slugs b. Bivalves - clams, oysters, and scallops c. Cephalopoda - squid, octopus, nautilus 3. This is the second largest animal phylum (Arthropods largest)!! Nearly 110,000 species, mostly marine. 4. This is most successful land animal phylum, believe it or not - 35,000 species of terrestrial mollusca, more than ALL other terrestrial animals combined! 4. The Rise of Segmentation i. Annelids represent prime examples of segmented worms 1. includes leeches, earthworms ii. Why segmentation? Allows specialization iii. Each segment can be specialized for different tasks (eating, sensory, sex, excretion, movement, etc.) iv. eventually, appendages became jointed - gave rise to Arthropods 5. Arthropods i. have an exoskeleton of chitin, similar to that in fungi ii. molt (shed exoskeleton) to increase in size iii. most successful animals on earth (over 1,000,000 species known) iv. comprise 2/3 of all known organisms (all higher kingdoms included) v. for example, one hectare football field size area )in the tropics may harbor 41,000 species of insects alone!! vi. includes insects, spiders (which are not insects!), crabs, lobsters 6. Echinoderms (starfish) i. these are mostly marine animals (6,000 species) ii. All have a 5 part body plan 7. Chordates i. about 43,000 species known ii. all have: 1. hollow nerve cord 2. notochord - cartilaginous stiffening where backbone should be; replaced by backbone in more advanced animals 3. pharyngeal gill slits - present in juvenile stages, but lost in adult forms 4. have an internal skeleton 5. all have a tail during time being an embryo, but most lose it during maturation 6. two major groups a. Acraniates - have no well defined brain i. sea squirts, tunicates, lancelets - most marine organisms ii. no true backbone, just a notochord b. Craniates - have a brain i. have backbones and brains |