Answer:
invertebrates
Step-by-step explanation:
The invertebrates lack a backbone. They include molluscs, annelida, arthropods and protozoa. They have a segmented body and well developed digestive system. Some are mostly found in sea, land or deserts. Some invertebrates lack a rigid exoskeleton. Some are pseudocoelum, possessing cavity between body wall. Some invertebrates have an incomplete digestive system consists of a digestive cavity with one opening which serves as both mouth and anus. Some invertebrates have a complex nervous system consisting of brain and several other sense organs.