Cnidaria

// Kingdom Animalia: Phylum Cnidaria ﻿// by Anna Korogodski

Cnidarians belong to the clade Radiata. Radiata consists of animals with **radial symmetry** (similar parts surrounding a central axis) and **diplobastic embryos**, meaning they have two cell layers. Thus all their cells are in contact with the environment, either the inside (**endoderm**) or the outside (**ectoderm**). //Important Cnidaria characteristics-// Cnidarians lack a **mesoderm**, a germ layer that develops into muscle, bone, cartilage, and blood. Therefore they do not contain organs. Cnidarians have a **gastrovascular cavity**, a pouch that is used for digestion and to distribute substances throughout their bodies. They also contain **cnidocytes**, which are unique cells that function as a defense mechanism and also help cnidarians capture prey. These cells are very important for the survival of cnidarians, and in fact, are what cnidarians are named after. //Body plan-// Cnidarians have a fairly simple body construction. Their basic body plan is a sac with a central digestive compartment, the **gastrovascular cavity**, and they have a single opening that functions as both the mouth and the anus. This body plan has two types: **sessile polyps** and **floating medusas**. **Sessile** means permanently attached; **sessile polyps** are cylindrical forms which are attached to the **substratum** (a layer of material in the ocean) by the part of their body that is opposite of their mouths. Meanwhile, their tentacles are extended upwards while waiting for prey. In contrast, **floating medusas** are flattened, "upside down" versions of polyps, meaning their tentacle dangle downwards. They have bell-shaped bodies and move freely through the water. A common example of a **floating medusa** is jellyfish. While some cnidarians are only **polyps** or only **medusas**, some pass through both a **polyp** stage and a **medusa** stage in their life cycle.
 * Diagnostic characteristics that define Cnidaria: **



//Classes-// Cnidarians are a diverse group with over 10,000 living species, most of which are marine. The classes of Cnidaria include:
 * Hydrozoa**-- most are marine, a few are freshwater, and they have both **polyp** and **medusa** stages in most species. Examples: Hydras, //Obelia//, Portuguese man-of-war, and some corals.
 * Scyphozoa**-- all are marine and the **polyp** stage is reduced. They are free swimming, and **medusas** are up to 2 m in diameter. Examples: jellyfish, sea wasp, sea nettle.
 * Anthozoa**-- all are marine and the **medusa** stage is completely absent. They are **sessile**. Examples: sea anemones, sea fans, most corals.

media type="youtube" key="vefspwrqJWI" width="425" height="350" align="center" (Video explains what jellyfish eat, what eat jelly fish, how jellyfish exchange gas, and types of locomotion of jellyfish. It is a quick summary. (SD) (5)
 * Acquiring and Digesting Food: **

Surprisingly, cnidarians are carnivores. **Cnidocytes**, specialized cells that are stored in their tentacles, help cnidarians to capture prey. They use their tentacles, which are arranged in a ring around their mouth, to capture prey and then push the food ﻿through their mouth, which is located at the top of a **polyp** and at the bottom of a **medusa** (at the center and in between the tentacles). This opening leads to the **gastrovascular cavity** which is enclosed in a body wall that is only two cells thick. This cavity is used for extracellular digestion and as a passageway to disperse materials throughout the cnidarian's body. Since digestion begins in the cavity, only the cells of the inner layer have direct access to nutrients, but the nutrients only have a short distance to diffuse to the cells of the outer layer. The elaboration of **gastrovascular cavities** varies in different cnidarians. Here is an image of the cnidarian //Aurelia// which has an elaborate **gastrovascular cavity**:
 * Gland cells** in the gastrovascular cavity secrete enzymes that aid in the complete digestion of food. **Gastromuscular cells** provide the raw power needed to digest the food. (NI) (4)



While the **gastrovascular cavity** functions in extracellular digestion, intracellular digestion is completed once small food particles are engulfed by **gastrodermal** cells. The **gastrodermis** is the inner layer of cnidarians and is also specialized for digestion.

Muscles and nerves occur in their simplest forms in cnidarians. Cnidarians lack a brain, but they have a non-centralized **nerve net** that is associated with sensory receptors. [The nerve net is located between the epidermis and the **mesoglea.** The mesoglea is a nonliving gelatinous material that separates the two tissue layers. (MP) [|2]These receptors are distributed radially around their bodies. This distribution enables cnidarians to detect and respond to stimuli equally from all directions. Cnidarians possess a statocyst, which is a gravitational sensory organ, and an ocellus, which is a simple photoreceptor organ. (DB) (1)
 * Sensing the environment: **



This is a nerve cell of a Cnidaria. The nerve net is between the epidermid and mesoglea, which is in between the epidermid and gastrodermis. (RG) [|(11)] Jelly Fish Anatomy (GR)[|14] A cnidarian's movement is coordinated by its **nerve net**-- nerve cells which connect the sensory and muscular cells. In some cases, such as the hydra, the neurons in its **nerve net** control the contractions and expansions of its cavity. Also, when a cnidarian closes its mouth, the volume of its cavity becomes securely placed, and thus the contraction of certain cells causes the cnidarian to change shape. This phenomenon occurs because most cnidarians have a **hydrostatic skeleton**. This type of skeleton consists of fluid held under pressure in a closed body compartment. Cnidarians use their muscles to change the shape of these fluid-filled compartments, thus enabling themselves to control their form and movement. Also, cells of their **epidermis** (outer layer) and **gastrodermis** (inner layer) have bundles of microfilaments that are arranged into fibers and are able to contract. While **sessile polyps** are attached to a material, **floating medusas** move freely through the water by passively drifting and contracting their bodies. Though **sessile polyps** don't have such a wide range of movement as the **floating medusas**, the polyps still move their parts, for example, when extending their tentacles.
 * Locomotion: **

Since cnidarians have a body wall that is only two cells thick, all their cells are in contact with the water. Each cell is able to absorb oxygen from the water in order to respire. **"**Respiration is basically accomplished by **diffusion**, which is the "spread of particles through random motion from regions of higher concentration to regions of lower concentration."**"** [MS] [|3] After water in the gastrovascular cavity has been depleted of oxygen, or stale, water is pumped out, to be replaced with new water, to improve respiration. (LJ)
 * Respiration: **

Cnidarians excrete through a single opening that serves as both their mouth and anus. Interestingly, the secretions of some cnidarians make up corals. In the class **Anthozoa**, which consists of polyps, coral animals secrete hard external skeletons made of calcium carbonate. These skeletal remains build up from each generation. This process ultimately constructs "rocks" with shapes characteristic of the species. Thus, it is these skeletons that we call coral.
 * Metabolic waste removal: **

The body plan of some cnidarians, such as hydra, makes a circulatory system unnecessary. This is because they have a **gastrovascular cavity** which functions in both digestion and the distribution of substances throughout the body, and also because all their cells are in contact with the environment. Circulation is mostly done though diffusion. (MLK)[|9]
 * Circulation: **

All 200 varieties of jellyfish have the ability to sting, which they use to paralyze the prey that comes within touch of their tentacles. The poison from the nematocysts of the jellyfish varies with the species of jellyfish. Some stings are very harmful to humans, while others will only cause minor skin irritation. (CC) ([|7])
 * Self protection: **
 * Cnidocyte** cells play a major role in self defense for cnidarians. **Cnidocytes** contain **cnidae**, which are capsule-like organelles that are capable of turning outward or inside out. Some **cnidae** are stinging capsules. These are called **nematocysts** and are the reason why jellyfish, for example, are able to sting. This ability is an important defense mechanism that can help to ward off predators.

Also, cnidarians' **hydrostatic skeletons** are well suited for life in aquatic environments. These skeleton provide protection by cushioning their bodies from shock. The outer layer of cnidarians, their **epidermis**, protects them as well. Cnidaria, interestingly, have the ability to regenerate entire parts of their body from a single cell if damaged. They can also reproduce asexually using a similar mechanism of regeneration. The Cnidaria "Hyrda" is named after the mythical creature with the same name that had the ability to grow two heads every time one head was cut off. (JS) [|(8)]

Both the inner and outer tissue layers of a cnidarian are bathed by fluid. Each cell is able to regulate how much water it diffuses because each cell is in contact with the water. Cnidarians do not have the ability to control/regulate water balance and ion levels.(CW)(6)
 * Osmotic Balance: **

Cnidarians are **ectotherms**, meaning cold-blooded animals. Their body temperature changes along with the water temperature.
 * Temperature Balance: **

Coral reefs, which temperature's vary with water temperature, have a temperature range from 23 to 29 degrees C, or 73 to 84 degrees F. Global warming could possibly have an extreme negative impact on coral reefs. In contrast, many jellyfish live in a large range of temperatures, from warm waters all over the globe to the chilly waters of the Arctic. (SI)(12)(13)

Reece, Jane B., and Neil A. Campbell. Campbell Biology. San Francisco: Pearson Benjamin Cummings, 2002. Print 1. [] 2. [] 3. [] 4. [] 5. [] 6. [] 7. http://www.bigsiteofamazingfacts.com/why-do-jellyfish-sting 8. [] 9. [] 10. [] 11. [|http://www.gwu.edu/~darwin/BiSc151/NoCoelom/no3.GIF&imgrefurl=&esq=4&page=2&ndsp=22&ved=1t:429,r:7,s:27&tx=128&ty=20] 12. http://www.earthlife.net/inverts/cnidaria.html 13. http://zipcodezoo.com/Key/Animalia/Cnidaria_Phylum.asp 14. http://www.google.com/imgres?imgurl=http://exotic-aquariums.com/images/Cross_section_jellyfish_en_%28edit%29.svg.png (GR) Review Questions: 1. By which process do Cnidarians respire and why do Cnidarians use this process? (AC) 2. Describe how cnidarians are able to move. (SM) 3. How are Cnidarians able to maintain their body temperature? (MF)
 * Sources: **