Simplify the following text for a Grade 9 native English-speaking person.
Original: Like all arthropods , arachnids have an exoskeleton , and they also have an internal structure of cartilage-like tissue called the endosternite , to which certain muscle groups are attached .
Simplified: Like all arthropods , arachnids have an exoskeleton . They also have an internal structure of cartilage-like tissue , to which certain muscle groups are attached . .