These invertebrates have simple digestive system, nervous systems and their skeletal and muscular systems are very primitive or non existent.
(These invertebrates; have; simple digestive system, nervous systems and their skeletal and muscular systems) (simple digestive system, nervous systems and their skeletal and muscular systems; are; very primitive)
These invertebrates simple digestive system, nervous systems and their skeletal and muscular systems
have are
simple digestive system, nervous systems and their skeletal and muscular systems
very primitive