# Lecture 12: Animal Diversity — Invertebrates I
## General Biology II — Organismal, Evolution & Ecology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the key features that define animals and the major branch points in animal phylogeny
2. Distinguish between diploblastic and triploblastic body plans, and the types of body symmetry
3. Describe the distinguishing characteristics of Porifera, Cnidaria, and Platyhelminthes
4. Compare acoelomate, pseudocoelomate, and coelomate body plans
5. Describe the biology of Mollusca and Annelida
6. Explain the ecological and medical significance of these invertebrate groups
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## Lecture Content
### I. Introduction to Animal Diversity
Animals (kingdom Animalia) are multicellular, heterotrophic eukaryotes that ingest food and digest it internally. Unlike plants and fungi, animal cells lack cell walls; instead, they are held together by structural proteins, most notably collagen, and by specialized cell junctions -- tight junctions that seal adjacent cells, desmosomes that anchor them mechanically, and gap junctions that allow direct chemical and electrical communication. Most animals are motile at some stage of their life cycle, and most reproduce sexually with a diploid-dominant life cycle. Embryonic development characteristically includes a blastula stage -- a hollow ball of cells -- and most animals possess nervous tissue and muscle tissue, the two tissue types that together enable the rapid, coordinated responses that define animal behavior.
The animal phylogenetic tree is organized around several major branch points. The first split separates sponges, which lack true tissues, from all other animals (Eumetazoa), which possess them. Among the eumetazoans, body symmetry divides the radially symmetrical groups (Cnidaria and Ctenophora) from the bilaterally symmetrical Bilateria. Bilaterians are triploblastic, possessing three germ layers (ectoderm, mesoderm, and endoderm), in contrast to the diploblastic cnidarians with only two. The nature of the body cavity further distinguishes bilaterians into acoelomates (no body cavity), pseudocoelomates (a cavity not fully lined by mesoderm), and coelomates (a true coelom fully lined by mesoderm). Finally, among the coelomates, a fundamental split separates the protostomes from the deuterostomes based on developmental differences.
In **protostomes**, the blastopore (the first opening of the developing gut) becomes the mouth, cleavage is spiral and determinate, and the coelom forms by splitting of mesodermal tissue (schizocoely). Protostomes include two major clades: the Lophotrochozoa (mollusks, annelids, and others) and the Ecdysozoa (arthropods, nematodes, and other molting animals). In **deuterostomes**, the blastopore becomes the anus, cleavage is radial and indeterminate, and the coelom forms by outpocketing of the archenteron (enterocoely). Deuterostomes include the Echinodermata and Chordata.
### II. Phylum Porifera (Sponges)
Sponges are the simplest animals, lacking true tissues, organs, and a nervous system. As adults, they are sessile filter feeders with a porous body plan built around the flow of water. Choanocytes (collar cells) lining the interior create water currents with their flagella and capture food particles from the passing stream. Water enters through numerous small pores (ostia), flows through the spongocoel (central cavity), and exits through a single large opening, the osculum. Between the outer and inner cell layers lies the mesohyl, a gelatinous matrix containing amoebocytes -- mobile cells that transport nutrients, secrete structural elements, and can differentiate into other cell types. Structural support comes from spicules made of calcium carbonate or silica, and/or from spongin, a flexible network of protein fibers.
Sponges reproduce both asexually (by budding, fragmentation, and the formation of gemmules -- resistant clusters of amoebocytes) and sexually. Most species are hermaphroditic, releasing sperm into the water column while retaining eggs for internal fertilization. Ecologically, sponges filter vast quantities of water, provide habitat for other organisms, and produce bioactive compounds that are of growing interest in pharmaceutical research.
### III. Phylum Cnidaria (Jellyfish, Corals, Anemones, Hydrozoans)
Cnidarians are radially symmetrical, diploblastic animals with true tissues but no organs. Their body wall consists of an outer ectoderm and inner endoderm separated by a gelatinous mesoglea. Two body forms characterize the group and often alternate within a single life cycle: the **polyp**, which is sessile and cylindrical with the mouth facing upward (as in sea anemones and coral), and the **medusa**, which is free-swimming and bell-shaped with the mouth facing downward (as in jellyfish).
The signature feature of cnidarians is the **cnidocyte**, a specialized stinging cell found nowhere else in the animal kingdom. Each cnidocyte contains a nematocyst -- a coiled, barbed, harpoon-like organelle that discharges explosively on contact, injecting toxins into prey or predators. Cnidarians possess a gastrovascular cavity with a single opening that serves as both mouth and anus, functioning in digestion and nutrient distribution. Their nervous system is a diffuse nerve net without a centralized brain.
The major cnidarian classes include the **Hydrozoa** (Hydra, Obelia, the colonial Portuguese man-of-war), **Scyphozoa** (true jellyfish such as Aurelia, with the medusa as the dominant life stage), **Cubozoa** (box jellyfish, including the extremely venomous Chironex fleckeri), and **Anthozoa** (sea anemones and corals, which exist exclusively as polyps). Reef-building (hermatypic) corals secrete calcium carbonate skeletons and harbor zooxanthellae -- symbiotic dinoflagellates that provide up to 90% of the coral's energy. Coral reefs are among the most biodiverse marine ecosystems on Earth but are severely threatened by bleaching, a process in which warming seas cause corals to expel their zooxanthellae.
A figure comparing cnidarian body forms and the cnidocyte mechanism. Panel A: Side-by-side comparison of the polyp (vase-shaped, mouth/tentacles facing upward, attached to substrate) and medusa (bell-shaped, mouth/tentacles facing downward, free-swimming) body forms, with labeled ectoderm, mesoglea, endoderm, gastrovascular cavity, and tentacles. Panel B: A magnified cnidocyte cell — the nematocyst is shown coiled inside the cell in the "loaded" state, with a trigger (cnidocil) on the cell surface. Upon stimulation, the nematocyst discharges, everting a barbed thread that penetrates prey and injects toxin. Before and after discharge states are shown side by side. Panel C: Representative cnidarians — a hydra (polyp), a jellyfish (medusa), a sea anemone (polyp), and a branching coral colony (polyps).
### IV. Phylum Platyhelminthes (Flatworms)
Flatworms are bilaterally symmetrical, triploblastic, acoelomate animals -- meaning that mesoderm fills the entire space between the ectoderm and endoderm, leaving no body cavity. Their dorsoventrally flattened shape is an adaptation that allows gas exchange to occur entirely by diffusion across the body surface, eliminating the need for dedicated respiratory or circulatory systems. Flatworms show cephalization, with sensory structures concentrated at the anterior end. They possess a gastrovascular cavity with a single opening (though tapeworms have lost the digestive system entirely) and excrete through protonephridia equipped with flame cells that regulate osmotic balance.
The major classes include the **Turbellaria**, mostly free-living flatworms exemplified by the freshwater planarian, which possesses auricles, eyespots (ocelli), and a remarkable capacity for regeneration. The **Trematoda** (flukes) are parasites with complex life cycles that typically involve a snail intermediate host. Schistosoma causes schistosomiasis, a blood fluke disease affecting more than 200 million people worldwide. The **Cestoda** (tapeworms) are intestinal parasites that have lost their digestive systems entirely, absorbing nutrients directly through their body covering (tegument). They attach to the host intestinal wall with a scolex bearing hooks and suckers, and their body consists of a chain of repeating segments called proglottids, each containing a complete set of reproductive organs. The pork tapeworm Taenia solium can reach several meters in length within the human intestine.
### V. Phylum Mollusca
The mollusks are the second most species-rich animal phylum, with approximately 100,000 described species. They are triploblastic coelomates and protostomes belonging to the Lophotrochozoa. Despite their enormous diversity, all mollusks share a body plan organized around three fundamental components: a **muscular foot** used for locomotion (whether crawling, digging, or jet propulsion), a **visceral mass** containing the internal organs, and a **mantle**, a tissue layer that secretes the shell (when present) and encloses the mantle cavity where gills are housed. Most mollusks possess a **radula**, a rasping tongue-like feeding structure (absent in bivalves). Most have an open circulatory system in which hemolymph flows through sinuses rather than enclosed vessels, though cephalopods have evolved a closed circulatory system.
The major classes display remarkable diversity. **Gastropods** (snails and slugs) have undergone torsion -- a developmental twisting of the visceral mass by 180 degrees -- and typically bear a single coiled shell (or none, in slugs). **Bivalves** (clams, mussels, oysters, scallops) possess a two-part hinged shell and are filter feeders that have lost the head and radula. **Cephalopods** (squid, octopus, cuttlefish, nautilus) are the most neurologically sophisticated invertebrates, with closed circulatory systems, jet propulsion, camera-type eyes, and chromatophores that enable rapid color change. Their shell is reduced (the pen of squid), internalized (the cuttlebone), or absent (octopus). **Polyplacophora** (chitons) are marine grazers bearing eight overlapping dorsal shell plates.
### VI. Phylum Annelida (Segmented Worms)
The annelids are triploblastic coelomates and lophotrochozoan protostomes whose defining feature is **segmentation (metamerism)** -- the division of the body into repeated segments, each separated by septa and each containing its own set of excretory organs (nephridia), nerve ganglia, and muscles. This modular organization allows independent movement of different body regions, greatly enhancing locomotion compared to unsegmented worms. Annelids have a closed circulatory system with blood flowing through vessels, exchange gases through their moist skin (supplemented by gills in some marine species), and use a hydrostatic skeleton in which the fluid-filled coelom serves as the medium against which muscles contract.
The major annelid classes include the **Polychaeta** (marine worms with fleshy, paddle-like parapodia bearing chaetae on each segment), the **Oligochaeta** (earthworms, with few setae and no parapodia, which are hermaphroditic and play vital roles in soil aeration and decomposition), and the **Hirudinea** (leeches, mostly freshwater ectoparasites or predators equipped with anterior and posterior suckers). Leeches produce hirudin, a powerful anticoagulant, and are used medicinally to reduce blood pooling after reconstructive surgery.
A comparative anatomy diagram of three mollusk classes. Panel A (Gastropod — snail): Side view showing the coiled shell, muscular foot, head with tentacles and eyes, visceral mass inside the shell, mantle, radula within the mouth, and ctenidia (gill) in the mantle cavity. Panel B (Bivalve — clam): Cross-section showing two hinged shell valves, the mantle lining each valve, the muscular foot extending ventrally, siphons (incurrent and excurrent) for water flow, gills for filter feeding and gas exchange, and the adductor muscles that close the shell. Panel C (Cephalopod — squid): Side view showing the head with large camera-type eyes, beak, eight arms and two longer tentacles with suckers, the mantle surrounding the visceral mass, a funnel (siphon) for jet propulsion, internal pen (reduced shell), and ink sac. Key features are labeled and color-coded by body region (foot, visceral mass, mantle).
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