2.1 lecture 2 10/20 (CNIDARIA and PLATYHELMINTHES)

Describe the diversity and habitat of Cnidarians.

Cnidarians are a diverse group of approximately 11,000 aquatic, mostly marine, invertebrates, including corals, jellyfish, sea anemones, and hydras. They are found in all ocean depths and environments, though a few genera like Hydra and certain jellyfish have adapted to freshwater.

How do Cnidarians compare in species diversity to Ctenophores?

Cnidarians comprise approximately 11,000 described species, demonstrating remarkable diversity, whereas ctenophores (comb jellies) discussed previously are a much smaller group, numbering in the hundreds of species.

What is the defining characteristic of all Cnidarians?

A defining characteristic of all cnidarians is the presence of specialized stinging cells called cnidocytes (or nematocytes), used for prey capture and defense.

Describe the structure and function of a Nematocyst and its activation mechanism.

Each cnidocyte houses a powerful, harpoon-like organelle called a nematocyst. These contain a coiled, thread-like tube armed with barbs and filled with potent toxins (neurotoxins, hemolysins, cytolysins). Upon activation, the nematocyst rapidly everts and injects venom. Activation involves both mechanical contact with a cnidocil (hair-like trigger) and specific chemical signals from prey, acting as a 'two-factor authentication' system.

Compare and contrast the two main body forms of Cnidarians: Polyp and Medusa.

The Polyp form is typically benthic (bottom-dwelling) and sessile (immobile), appearing as a cylindrical body attached to a substrate with an upward-pointing mouth and tentacles (e.g., sea anemones, coral polyps). The Medusa form (jellyfish) is pelagic (free-swimming) and mobile, with an umbrella or bell shape, and mouth/tentacles usually hanging downwards, adapted for a planktonic lifestyle via rhythmic bell contractions.

Explain the characteristics of the Cnidarian digestive system.

Cnidarians possess a simple, sac-like gastrovascular cavity that functions for both digestion and circulation of nutrients. This 'incomplete' digestive system has only one opening that serves as both the mouth (for ingesting food) and the anus (for expelling undigested waste). Digestion occurs both extracellularly within the cavity and intracellularly within its specialized lining cells.

What are the common names and key characteristic of the class Anthozoa?

Class Anthozoa ('flower animals') includes sea anemones, corals, and sea pens. It is the largest class (over 6,000 species) and unique for exclusively exhibiting the polyp stage throughout their entire life cycle, without a free-swimming medusa form. They reproduce both asexually and sexually. Corals are colonial polyps secreting calcareous skeletons forming reefs.

What are characteristics of the class Hydrozoa, including examples?

Class Hydrozoa is highly diverse (3,000-4,000 species) and often exhibits an alternation of polyp and medusa stages, though some species specialize in one form. Examples include solitary freshwater hydras (polyp) and colonial marine organisms like the Portuguese man o' war (Physalia physalis), which is a floating colony of specialized polyps.

Describe the characteristics of the class Scyphozoa.

Class Scyphozoa ('cup animals') encompasses most large, familiar jellyfish (approx. 200 species). In scyphozoans, the medusa form is the dominant and most conspicuous stage of the life cycle, while the polyp stage is usually very small, short-lived, or absent. They are characterized by rhythmic bell pulsations for locomotion.

What are the key features and dangers associated with the class Cubozoa?

Class Cubozoa are known as box jellyfish due to their cube-shaped bell (around 50 species). They are infamous for their highly developed eyes and extremely potent neurotoxins. Their venom can be rapidly fatal to humans, causing severe pain, cardiac arrest, and respiratory failure, particularly in Indo-Pacific species.

Explain the evolutionary pattern of life cycles in Anthozoans versus other Cnidarian classes.

Anthozoans represent a more ancestral or simplified life cycle, existing exclusively in the sessile polyp form with direct development or larval dispersal. Hydrozoans, Scyphozoans, and Cubozoans typically exhibit a complex life cycle with alternation of generations: a sessile polyp stage (asexual reproduction via budding) and a pelagic medusa stage (sexual reproduction via gamete release). This biphasic cycle allows for localized proliferation and wide dispersal, offering adaptive advantages.

Provide specific examples of Hydrozoans and their unique characteristics.

Hydrozoans include the solitary freshwater Hydra, existing solely as a polyp. The Portuguese Man o' War (Physalia physalis) is a colonial marine organism, not a single individual, composed of specialized polyps working together for floating, feeding, and reproduction, known for its potent stings.

Characterize Scyphozoans and Cubozoans based on their dominant life stages and habitats.

Scyphozoans (jellyfish) are characterized by a dominant medusa form with a greatly reduced polyp stage, making the free-swimming jellyfish the most prominent life stage, often found in open ocean environments. Cubozoans (box jellies) have a distinctive cube-shaped bell and are known for highly toxic stings, predominantly inhabiting tropical and subtropical waters, especially in Southeast Asia and Northern Australia.

What is Radial Symmetry and what organisms typically exhibit it?

Radial Symmetry characterizes organisms that can be divided into equal halves by more than two planes passing through the central axis. This body plan is advantageous for sessile or slow-moving organisms, such as cnidarians, allowing them to sense their environment and capture prey from all directions.

What is Bilateral Symmetry and what evolutionary advantages does it confer?

Bilateral Symmetry means an organism can only be divided into two mirror-image halves along a single sagittal plane. This body plan is found in more complex animals and is strongly associated with active, directed movement and the evolution of cephalization, leading to differentiation of distinct anterior/posterior, dorsal/ventral, and left/right sides.

Define Cephalization and explain its significance.

Cephalization refers to a major evolutionary trend where nervous tissue, sensory organs (e.g., eyes, antennae, chemoreceptors), and the mouth are concentrated at the anterior (head) end of the body. This is a direct consequence of bilateral symmetry and active, forward-directed locomotion, enhancing an animal's ability to detect prey, predators, and mates, thereby improving survival and feeding efficiency.

Distinguish between Diploblastic and Triploblastic organisms in terms of embryonic germ layers.

Diploblastic organisms (e.g., cnidarians, ctenophores) develop from two primary germ layers: the ectoderm (outer) and endoderm (inner), lacking a true mesoderm. Triploblastic organisms (e.g., flatworms, and all bilaterally symmetrical animals) develop from three primary germ layers: the ectoderm, endoderm, and a distinct mesoderm layer situated between them.

What organs and structures are derived from the Ectoderm?

The Ectoderm is the outermost germ layer, responsible for forming the outer covering (epidermis), the nervous system (brain, spinal cord, nerves), and sensory organs (eyes, ears, taste buds). Examples: Epidermis, brain, spinal cord, nerves, cornea, lens of eye.

What organs and structures are derived from the Mesoderm?

The Mesoderm is the middle germ layer, which differentiates into musculature, the skeletal system (bones and cartilage), the circulatory system (heart, blood vessels, blood), the excretory system (kidneys), the reproductive system, and most connective tissues. Examples: Muscles, bones, cartilage, blood, blood vessels, heart, kidneys, gonads.

What organs and structures are derived from the Endoderm?

The Endoderm is the innermost germ layer, which gives rise to the lining of the digestive tract (gut), as well as the lining of associated organs such as the liver, pancreas, and the respiratory system (lungs in vertebrates). Examples: Lining of the digestive tract, liver, pancreas, lining of the respiratory tract.

What are Flatworms (Phylum Platyhelminthes) and why are they significant in evolution?

Flatworms (Phylum Platyhelminthes) are simple, bilaterally symmetrical, triploblastic invertebrates. They are notable for representing an early evolutionary lineage with true organs and an excretory system. They also serve as a key group for understanding the evolution of parasitism, with many classes showing highly adapted complex life cycles involving multiple hosts.

List and briefly describe the four classes of Flatworms.

The four classes of Flatworms are:

1. Turbellarians: Mostly free-living, non-parasitic, found in various aquatic/moist terrestrial environments; predators/scavengers with ciliated epidermal cells for locomotion.
2. Monogeneans: Typically ectoparasites, primarily on fish skin/gills, with simple direct life cycles involving one host and a haptor for attachment.
3. Trematodes (Flukes): All endoparasites with complex life cycles involving multiple hosts (typically a mollusc intermediate and a vertebrate definitive host); flattened, leaf-shaped bodies with oral and ventral suckers.
4. Cestodes (Tapeworms): Highly specialized endoparasites of the vertebrate digestive tract, lacking a mouth/digestive system, absorbing nutrients directly. Consist of a scolex for attachment and reproductive segments called proglottids.

Describe the life cycle and public health impact of Schistosoma (blood flukes).

Schistosoma causes schistosomiasis (bilharzia). Its complex life cycle involves humans as the definitive host (sexual reproduction) and a specific freshwater snail as the intermediate host (asexual reproduction). Infective larvae (cercariae) released from snails penetrate human skin. Schistosoma has a profound public health impact in tropical/subtropical regions, with transmission linked to contaminated water sources. Prevention involves sanitation and safe water access.

Explain the specialized structure and life cycle of Cestodes (Tapeworms).

Cestodes are specialized for an endoparasitic lifestyle. The scolex (head) has suckers/hooks for firm attachment to the intestinal wall of the definitive host. Lacking a mouth and digestive tract, they absorb nutrients directly across their tegument. The body consists of a chain of proglottids, which bud from the neck. Each mature proglottid contains complete male and female reproductive organs; gravid proglottids filled with eggs detach and are shed in feces.
Their life cycle involves multiple hosts: eggs in the definitive host's feces are ingested by an intermediate host (invertebrate/vertebrate), developing into larval stages (e.g., cysticerci). The definitive host is infected by consuming raw/undercooked intermediate host tissues containing these larvae.

Summarize the significance of complex life cycles and parasitism.

The evolution of complex life cycles in parasitic organisms significantly increases the diversity and intricacy of interactions within host systems, often involving host manipulation and co-evolution. These cycles allow parasites to exploit multiple niches and ensure transmission. Effective protections against parasitic infections include diligent proper sanitation practices, thoroughly cooking food to destroy infective cysts/larvae, and ensuring access to safe, clean drinking water. Understanding these relationships has immense implications for human health, livestock management, and broader ecosystem dynamics.

What is the definition of a 'Complex Life Cycle' in the context of Cnidarians and Flatworms?

In many cnidarians (Hydrozoans, Scyphozoans, Cubozoans) and parasitic flatworms (trematodes, cestodes), a Complex Life Cycle refers to a life history involving an alternation of generations or multiple host organisms. It typically includes both asexual (e.g., polyp budding) and sexual (e.g., medusa gamete release) reproductive stages, often exploiting different ecological niches or host species for transmission.

What is Zooxanthellae and its ecological importance?

Zooxanthellae are symbiotic photosynthetic algae (dinoflagellates), primarily Symbiodinium species, that live within the tissues of certain marine invertebrates, most notably corals. They provide the host with nutrients through photosynthesis, contributing significantly to coral growth and reef formation, making them vital to marine ecosystems.

Define Coral Bleaching.

Coral Bleaching is the process where corals expel their symbiotic zooxanthellae due to stress (e.g., increased water temperature, pollution). This causes the coral to lose its color and often leads to its death if the stress is prolonged, severely impacting coral reef health.