Exoskeletons Arthropods and certain mollusks have exoskeletons. Molluscan exoskeletons are made of protein strengthened with mineral salts. Arthropods have exoskeletons made of chitin . Figure 44.14 illustrates muscle attachments in arthropod exoskeletons. Endoskeletons Vertebrates have endoskeletons of cartilage, bone, or a combination of both.
Cartilage is more flexible than bone.
Osteoblasts lay down new bone matrix and become surrounded by the matrix. Once surrounded, osteoblasts become the maintenance cells called osteocytes . Osteoclasts break down bone matrix and help recycle calcium and repair and reshape bone. Figure 44.15 illustrates the human skeleton. Figure 44.17 illustrates endochondral bone formation in a long bone.