Inverterbate animals do not have internal supporting structures or skeletons.

Instead, invertebrates can have durable, external supporting structures. This is the case with all insects, crabs and mussels. 

Water-dwelling invertebrates do not need these kinds of external supporting structures, because the water provides enough support to their bodies. For example, the octopus (pictured on the right) does not need an internal skeleton or an external shell to maintain its structure in the water. 

Invertebrates do not have highly developed senses. Their circulatory and respiratory systems are likewise poorly developed.