Classification of sponges is carried out on the basis of their reinforcing structures. Such type of structures, in case of calcareous sponges, are known as spicules. Spicules are made up of calcium carbonate and form a skeleton of calcareous sponges. They are just 10cm in length; however, they encompass different kinds of structures like:

• Asconoid (Indefinite shape)
• Syconoid (Tubular body)
• Leuconoid (Flagellated chambers)

The demosponges, just like calcareous sponges are made up of spicules; however, in demosponges, spicules are formed of fibers produced from a protein called 'spongin'. They are largest of all sponges and form 90% of their total number. Demosponges live in warm as well as cold areas of oceans. Some of the demosponges reproduce sexually while the others asexually. These organisms exhibit great variety of form and are much more diversified than other classes of sponges. Asexual reproduction takes place by means of gemmules and budding. Gemmules are internal buds present in sponges that are capable of developing in favorable conditions; they remain dormant in the absence of favorable conditions. According to Mr. Gordon Love, a scientist from California University, demosponges came into existence as early as 635 million years ago i.e. during the Neoproterozoic era. The different types of glass sponges make up the class Hexactinellida. These sponges are of rare occurrence in nature. Generally, they are found at great depths (450-900m) in oceans. Their body is cup-shaped; the spicules present on their body are made up of siliceous compounds. Glass sponges can live for 15,000 years which places them amongst longest living creatures on earth (Posidonia oceanica, a seagrass is the longest living organism with a lifespan of about 200,000 years). The glass sponges are very sensitive to external stimuli. They are capable of reacting quickly owing to the electric impulse they generate. There is another criterion for classifying the glass sponges, which is their ability to build reefs. Different types of glass sponges are described below.

• Reef-Building Sponges: They are formed of funnel-shaped Heterochone calyx or Finger Goblet Sponge which grows up to 1.5m in height. Reef-building sponges have shades of yellow and white. Other two species of reef-builders are the Aphrocallistes vastus, also known as Cloud Sponge and Farrea occa. The unique characteristic of Cloud sponges is their mitten-like outgrowth. They are funnel-shaped and have a height of 2m. Farrea occa is the biggest of all reef-building sponges and attains a height of up to 15m.
• Non-reef Building Sponges: There are four species of non-reef builders that are white in color and which attain a height of up to 50cm. Staurocalyptus dowlingi, Acanthascus platei, Acanthascus cactus and Rabdocalyptus dawsoni are the different species of non-reef builders.

The different forms of sponges commonly found in nature are described below. Vase Sponge: Sponges from this group are mainly found in the region surrounding the Caribbean islands and the coast of Florida, USA. The vase sponges can grow up to 40-60cm in diameter. This kind of sponge is found in different shades of brown, red and purple. Scientific name of this sponge is Ircinia campana.

• Azure Vase Sponge: The Azure Vase Sponge is characterized by a tubular and vase-like shape. Its outer surface is marked with pits and network of grooves. These sponges can grow up to 30cm in length. Pits found on the surface of Azure Vase Sponges are 0.5 to 1cm in diameter.
• Strawberry Vase Sponge: These sponges are mainly found in areas surrounding the Caribbean islands. The strawberry vase sponges are found in oceans at a depth of 15-40 meters.

Barrel Sponge: The barrel sponge can grow up to 1.5m in length. This kind of sponge is known by names like Volcano Sponge, Siliceous Sponge and Great Barrel Sponge. An adult human being can easily fit into the barrel-shaped structure of this sponge. One of the special features of this sponge is that its surface is marked with deep ridges. Tube Sponge: The tube sponge, as the name suggests, is formed of thick-walled and tube-like structures that are connected at the base. The kind of sponge grows up to 1m in length. The stovepipe sponge, a type of tube sponge possesses a tubular body. It grows up to 150cm in length. The shape and structure of this kind of sponge is similar to tube sponges. All the tubes of this sponge, which constitute a stovepipe structure, are connected at a single, narrow base. These sponges are known to release toxic chemicals to deter their predators. Orange Ear Elephant Sponge: This demosponge is found in areas surrounding the Caribbean islands, Florida (USA) and Bahamas. Surface of this demosponge is reddish orange in color. However, beneath the surface, its orange color is paler in comparison to that of the outer skin. Color of the orange ear elephant sponge varies greatly. One can also find shades of brown and red in the orange elephant sponges. These sponges are lobate to rounded in shape and their width ranges from 5-30cm. Row Pore Rope Sponge: Scientific name of this sponge is Aplysina cauliformis. It has different shades like brown, purple-violet, cream, green, cinnamon-tan, etc. The row pore rope sponge is mainly found in Bahamas near islands like Cat Island, Little Inagua, Great Island and San Salvador.