The polyps’ tentacles - just like those of jellyfish and anemones - are thick with bulbous cells (called cnidoblasts) containing a thin hollow filament that is sharp and coiled like a spring (the nematocyst). The moment they contact prey, cnidoblasts open automatically and jab their nematocysts lightning fast into the victim’s tissue, injecting a toxic compound.
Soft corals (Alcyonacea) belong together with gorgonians to the subclass Octocorallia. The colony of polyps in these organisms - often of blazing beauty - is supported by a fleshy body whose texture is sometimes leathery (as in Sarcophyton and Sinularia) and other times flabby and semi-transparent. It is often “inflated” at will when a current is strong and rich in nutrients (as in the genus Dendronephthya). In the latter case, the central structure is reinforced by numerous calcareous spicules. Soft corals defend themselves from predators by secreting toxic substances and hosting a multitude of guest organisms.
The tentacles of the polyp then carry the dead or paralyzed victim toward the oral cavity and ingest it. Up close at night, it is easy to observe the tiny distended polyps that stud the surface of a coral colony by the thousands, projecting their tentacles into the current and waiting for likely prey.
Madreporic colonies assume completely different shapes depending upon the species that reside in them. In a typical coral reef it is fairly easy to find colonies that are laminate in structure (the classical tabular madrepores of the Acropora genus), fingerlike (with short and stubby branching characteristics, as in the Montipora genus), branching (Acropora again), massive (the giant rounded “loaves” of the Porites, Favites and Favia genera, among others), brain-shaped or labyrinthine (Diploria, Platygira, Turbinaria) and even non-sessile (that is, detached from the substrate, as in the flat round colonies of the mushroom corals belonging to the Fungia genus).
In any case, every structure responds to precise conditions and often presents a useful refuge for a great number of different species such as fish and invertebrates.
With more than 10,000 species, almost all of them marine, sponges represent one of the most important groups of invertebrates. They compete with corals in the better lit sections of the reef and occupy specific ecological niches at greater depths. These are filtering animals that suck the surrounding water into their bodies through countless tiny holes (invisible to the naked eye) and then expel it through a smaller number of bigger holes (called oscula which are easier to discern. Reef sponges vary enormously in size and shape and many prove surprisingly rigid and “fiberglassy” to the touch.
The polyps’ tentacles - just like those of jellyfish and anemones - are thick with bulbous cells (called cnidoblasts) containing a thin hollow filament that is sharp and coiled like a spring (the nematocyst). The moment they contact prey, cnidoblasts open automatically and jab their nematocysts lightning fast into the victim’s tissue, injecting a toxic compound. 
Madreporic colonies assume completely different shapes depending upon the species that reside in them. In a typical coral reef it is fairly easy to find colonies that are laminate in structure (the classical tabular madrepores of the Acropora genus), fingerlike (with short and stubby branching characteristics, as in the Montipora genus), branching (Acropora again), massive (the giant rounded “loaves” of the Porites, Favites and Favia genera, among others), brain-shaped or labyrinthine (Diploria, Platygira, Turbinaria) and even non-sessile (that is, detached from the substrate, as in the flat round colonies of the mushroom corals belonging to the Fungia genus). 
