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Protruding into the open space inside the feeding chamber is a mesh-like collar composed of microvilli with a single flagellum in the center of the column. This is formed of both living and non-living layers. Endoskeleton is found in vertebrates whereas exoskeleton is found in arthropods. Gemmules are capable of attaching to a substratum and generating a new sponge. Red bone marrowalso called myeloid tissuecontains hemopoietic stem cells, which produce an assortment of different blood cells through haematopoiesis. Hydrostatic skeleton. A hydrostatic skeleton is formed by a fluid-filled compartment held under hydrostatic pressure; movement is created by the muscles producing pressure on the fluid. Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles. For example, epithelial-like cells called pinacocytes form the outermost body, called a pinacoderm, that serves a protective function similar that of our epidermis. These osteocytes are connected to each other in a network of tiny canals called canaliculi, which allows them to transport minerals, fatty acids and waste and between each other. Firstly, as the arthropod grows, a steroidal hormone called ecdysone is released into the body; this signals the beginning of the molting process. An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms. C) Lungs. The innermost layer is the hypostracum or nacreous layer. The spine of the scapula is a good example of a bony protrusion that facilitates a broad area of attachment for muscles to bone. Each toe consists of three phalanges, except for the big toe that has only two (Figure 19.15). The human skeleton is an endoskeleton that is composed of the axial and appendicular skeleton. A) Tetrapod locomotion. The radius is located along the lateral (thumb) side of the forearm and articulates with the humerus at the elbow. Members of the phylum ctenophores like Ctenoplana, Velamen, Beroe, etc. Each of the five classes has distinguishing characteristics that allow members to be classified appropriately. By the end of this section, you will be able to do the following: As we have seen, the vast majority of invertebrate animals do not possess a defined bony vertebral endoskeleton, or a bony cranium. The skeleton of the red-knobbed sea star (Protoreaster linckii) is an example of a hydrostatic skeleton.