They have porous bodies with water canals in the body wall. There are three body types that are currently recognized, that is: 1) asconoid, 2) syconoid and 3) leuconoid, the simplest of the three being the asconoid and the most complex being the leuconoid. Water enters through pores or canals in these organisms into the central cavity known as the spongocoel and flows out through the osculum, carrying along with it any waste products. Epidermal cells line the outer layer of the sponge and the canals. Between the outer and inner cell layers lies the mesohyl, supported by spicules which are like slender skeletal spikes made of calcium carbonate or silica. Spongin, a form of collagen, forms the fiberous part of the spongal skeleton. In sponges, the absence of specialized nerve cells and response organs mean that reaction to stimuli is dependent on individual cells. Cnidarians are capable of locomotion and have cells which are more associated than the sponges. They have a two body shapes, the polyp and medusa. Their germ layers are separated by mesoglea, a thick, jelly-like layer, and they contain a central digestive compartment called gastrovascular cavity …show more content…
Cnidocytes, which are specialized cells located mainly in the epidermis and contain nematocysts, special stinging organelles, to capture their prey or protect them from predators. Each cnidocyte has a small, projecting trigger and a hollow thread inside, which may be sticky or contain poisonous barbs or spines that paralyze their prey, preventing them from escaping. The contractile cells form contractile fibres, which allow for locomotion by the formation of a hydrostatic skeleton. Together with the nerve cells, they function as response organs as well. Nerve cells transmit impulses throughout the body of the cnidarian, allowing it to detect stimuli evenly from all