Each type utilizes actin and myosin to bring about contraction. We will now consider further some of the anatomical features of the skeletal muscle fibers as well as their function. The current theory used to explain muscle fiber contraction is called the sliding filament theory. 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 .