Sidney H Reynolds

The Vertebrate Skeleton Sidney H Reynolds

Publisher Marketing: By the term skeleton is meant the hard structures whose function is to support or to protect the softer tissues of the animal body. The skeleton is divisible into A. The Exoskeleton, which is external; B. The Endoskeleton, which is as a rule internal; though in some cases, e.g. the antlers of deer, endoskeletal structures become, as development proceeds, external. In Invertebrates the hard, supporting structures of the body are mainly exoskeletal, in Vertebrates they are mainly endoskeletal; but the endoskeleton includes, especially in the skull, a number of elements, the dermal or membrane bones, which are shown by development to have been originally of external origin. These membrane bones are so intimately related to the true endoskeleton that they will be described with it. The simplest and lowest types of both vertebrate and invertebrate animals have unsegmented skeletons; with the need for flexibility however segmentation arose both in the case of the invertebrate exoskeleton and the vertebrate endoskeleton. The exoskeleton in vertebrates is phylogenetically older than the endoskeleton, as is indicated by both palaeontology and embryology. Palaeontological evidence is afforded by the fact that all the lower groups of vertebrates-Fish, Amphibia, and Reptiles-had in former geological periods a greater proportion of species protected by well-developed dermal armour than is the case at present. Embryological evidence tends the same way, inasmuch as dermal ossifications appear much earlier in the developing animal than do the ossifications in the endoskeleton. Skeletal structures may be derived from each of the three germinal layers. Thus hairs and feathers are epiblastic in origin, bones are mesoblastic, and the notochord is hypoblastic.