An invertebrate is an animal with no back bone.

Animals are grouped into two groups: vertebrates or invertebrates.

A vertebrate is an animal that does have a back bone. 

People, cats and dogs have back bones, so we are all vertebrates.

Insects do not have back bones. That means all insects are invertebrates.

Those other bugs we've studied, like spiders and slugs, are also invertebrates.

Fresh water changes how much buoyancy the shark’s liver can provide. The researchers found that when sharks swim in fresh water, they are two to three times less buoyant than when they swim in salt water. In order to keep afloat the same way they do in salt water, sharks would need livers eight times as large—unrealistic for an organ that’s already the largest in a shark’s body.

Some kinds of stingrays have a spine in their tail with a very sharp point and edges that are serrated or notched.
Many species, including the exotic-looking blue-spotted stingray, have venom that is delivered through their tails. That venom, and the spine itself, can be dangerous to humans.

Their colors usually match the mud or sand they hide in.

While a stingray's mouth is on the underside of its body, along with its nostrils and gill slits, its eyes are on the top. Scientists don't think they use their eyes for hunting very much though. Instead, they use special sensors which can detect the tiny electrical charges emitted by their prey.​

Manta Rays
Shark's Sense of Smell