Sea urchins are spiny invertebrates from the phylum Echinodermata. They are the main species in the class Echinoidea. Their main habitat is upon the rocky surfaces on the bottom of the ocean floor. There are more than 700 different known kinds of these urchins. 
The urchin was created on the fifth day of creation. It is a species, along with all other creatures of the sea, that has only been around for a few thousand years, rather than, as evolutionists say, a million years. This is proved in Genesis 1:20-23 (KJV) which states: "And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl [that] may fly above the earth in the open firmament of heaven. And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that [it was] good.And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth. And the evening and the morning were the fifth day." 
The urchin is most active at night, though, during the day, it seems to eat constantly. 
The name "urchin" comes from their body's close resemblance to the spine-covered hedgehog. The Echinoidea's radial symmetrical body is composed of sharp, and often times venomous, spines that stick out in all directions from the shell (also known as the test). These spines help them to resist danger from predators. No matter what type of urchin it is, the sharp spines will put a stinging pain in whatever attempts to harm it.
Sea urchins come in various different sizes. The adults generally have a size ranging from about 3-10 cm, with their spines about 2 cm in length. Some urchins, however, can have sharp spines that are able to reach up to 20 cm in length. Spines that cover the urchins body are capable of moving in any direction, no matter what the species, but the length, sharpness, and color of these spines are dependent upon the type of urchin it is. The color of the urchin may vary from green to black to red.
They have a radial symmetrical body (though the larvae takes on a bilateral form of symmetry) that is constructed mainly of plates that line the inside of its ball-like form. The plates hold the echinoidea's body form together. Urchins transport themselves along the ocean floor with tube-like feet, which all animals in this class possess. The tube feet allow the animal to move itself in order to capture food, such as algae - its main source of food. They also help it to burrow itself into the sand to hide from it's predators.
The mouth of the Echinoidea, often referred to as Aristotle's Lantern because of the way he first described it, is located at the central most part of the animal's circular, spiny body. It is bordered by five sharp teeth that aid in the consumption of food. In the center of the razor-sharp teeth lies the urchin's tongue. Beyond the tongue is its esophagus and stomach, which is divided into five separate parts, where it eventually meets the anus of the animal. 
Sea urchins do not have brains. Instead, they use their spines as a way of knowing what is going on around them.
The top side of the urchin contains the anus and it's genetal plates. This is what makes the animals in its phylum unique. Most other animals would have the anus located on its underside. Five genetal plates along the the boarders of the anus, each containing a single hole called gonopores, help the urchin when it comes time to reproduce. 
The sea urchin feeds mostly upon algae. This keeps the algae population in check because it is their main source of food. Other foods that the urchin eats include barnacles (hard shells usually found on rocks), kelp, and dead or decaying animal matter. 
Smaller sea urchins will often eat the larger ones because they can move around more quickly to get than the big urchins.
Sea urchins reproduce sexually, meaning the animal performs a sexual activity in order to reproduce. When the time comes for the animal to reproduce itself, it does so by releasing numerous amounts of eggs and sperm - depending upon if the urchin is female or male - into the ocean through five holes in the genital plate referred to as "gonopores". These gonopores are located along the border of its anus. The egg meets with the sperm and becomes fertilized. After several months, the larvae is produced. It will, at first, have a body in the form of bilateral symmetry, whereas the adult symmetrical form is radial. The entire process of fertilization may last anywhere from 2 - 5 years before the urchin is completely formed into an adult. 
Though it is often assumed to be a sessile object at first glance, planted to the ocean floor like the sea sponge, urchins are actually very responsive to their environment and environmental changes. They are, in fact, mobile animals. The urchin's tube feet carry it slowly across the ocean floor to get to its next destination.
If something does so much as lightly skim the top of the urchin's spines, the spines will point in the direction in which they were touched. Sometimes it will react by either leaving the scene or taking the object into it's mouth and consuming it with its sharp teeth.
The sometimes spiky, but very sensitive spines serve the animal as protection from its predators, such as the sea otter, the starfish, or the crab. If the urchin is alarmed or frightened by the quick movements of its predator, it will use its tube feet to attempt to bury itself under the sand, hoping to not be seen by the intruder. 
Red Slate Pencil Sea Urchin
Red Pencil Slate Urchin
- Sea Urchin Wikipedia
- Sea Urchin Enchanted Learning Software
- Class Echinoidea Animal Diversity Web
- Echinoderms: Part 7 Live Aquaria
- BLB (KJV) Gen 1 Blue Letter Bible