Axial skeleton in the context of "Caudal fin"

The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans, it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of the clavicle, scapula, and coracoid. Some mammalian species (such as the dog and the horse) have only the scapula.

The pectoral girdles are to the upper limbs as the pelvic girdle is to the lower limbs; the girdles are the part of the appendicular skeleton that anchor the appendages to the axial skeleton.

Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates, including the non-vertebrate members of the phylum Chordata such as lancelets, sea squirts and salps. As the name suggests, marine invertebrates lack any mineralized axial endoskeleton, i.e. the vertebral column, and some have evolved a rigid shell, test or exoskeleton for protection and/or locomotion, while others rely on internal fluid pressure to support their bodies. Marine invertebrates have a large variety of body plans, and have been categorized into over 30 phyla.

Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and lift, which help the fish swim. Apart from the tail or caudal fin, fish fins have no direct articulations with the axial skeleton and are attached to the core only via muscles and ligaments.

Fish fins are distinctive anatomical features with varying internal structures among different clades: in ray-finned fish (Actinopterygii), fins are mainly composed of spreading bony spines or "rays" covered by a thin stretch of scaleless skin, resembling a folding fan; in lobe-finned fish (Sarcopterygii) such as coelacanths and lungfish, fins are short rays based around a muscular central bud internally supported by a jointed appendicular skeleton; in cartilaginous fish (Chondrichthyes) and jawless fish (Agnatha), fins are fleshy "flippers" supported by a cartilaginous skeleton. The limbs of tetrapods, a mostly terrestrial clade evolved from freshwater lobe-finned fish, are homologous to the pectoral and pelvic fins of all jawed fish.

A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal frame to which the organs and soft tissues attach; and the hydroskeleton, a flexible internal structure supported by the hydrostatic pressure of body fluids.

Vertebrates are animals with an endoskeleton centered around an axial vertebral column, and their skeletons are typically composed of bones and cartilages. Invertebrates are other animals that lack a vertebral column, and their skeletons vary, including hard-shelled exoskeleton (arthropods and most molluscs), plated internal shells (e.g. cuttlebones in some cephalopods) or rods (e.g. ossicles in echinoderms), hydrostatically supported body cavities (most), and spicules (sponges). Cartilage is a rigid connective tissue that is found in the skeletal systems of vertebrates and invertebrates.

The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmented column of vertebrae that surrounds and protects the spinal cord. The vertebrae are separated by intervertebral discs in a series of cartilaginous joints. The dorsal portion of the spinal column houses the spinal canal, an elongated cavity formed by the alignment of the vertebral neural arches that encloses and protects the spinal cord, with spinal nerves exiting via the intervertebral foramina to innervate each body segment.

There are around 50,000 species of animals that have a vertebral column. The human spine is one of the most-studied examples, as the general structure of human vertebrae is fairly typical of that found in other mammals, reptiles, and birds. The shape of the vertebral body does, however, vary somewhat between different groups of living species.

The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate.

In the human, the skull comprises two prominent parts: the neurocranium and the facial skeleton, which evolved from the first pharyngeal arch. The skull forms the frontmost portion of the axial skeleton and is a product of cephalization and vesicular enlargement of the brain, with several special senses structures such as the eyes, ears, nose, tongue and, in fish, specialized tactile organs such as barbels near the mouth.

The human skeleton is the internal framework of the human body. It is composed of around 270 bones at birth – this total decreases to around 206 bones by adulthood after some bones get fused together. The bone mass in the skeleton makes up about 14% of the total body weight (ca. 10–11 kg for an average person) and reaches maximum mass between the ages of 25 and 30. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton is formed by the vertebral column, the rib cage, the skull and other associated bones. The appendicular skeleton, which is attached to the axial skeleton, is formed by the shoulder girdle, the pelvic girdle and the bones of the upper and lower limbs.

The human skeleton performs six major functions: support, movement, protection, production of blood cells, storage of minerals, and endocrine regulation.