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Endoskeleton THE EVOLUTION OF THE VERTEBRATE ENDOSKELETON AN ESSAY ON THE SIGNIFICANCE AND) MEANING OF' SEGMENTATION IN COELOMATE ANIMIALS By W1T. 13. PRIMIROSE, M.B.. Cii.B. Lately Seniior Deniionistrator of lAnatomtiy i? the (nWizversity (of Glasgozc THE EVOLUTION OF THE VERTEBRATE ENDOSKELETON WVHEN investigating the morphology of the vertebrate head, I found it necessary to discover the morphological principles on which the segmentation of the body is founded. This essay is one of the results of this investigations, and its object is to show what has determined the segmented form in vertebrate animals. It will be seen that the segmented form in vertebrates results from a condition which at no time occurs in vertebrate animals. This condition is a form of skeleton found only in animals lower in the scale of organisation than vertebrates, and has the characters of a space containing water. This space is the Coelomic Cavity. The coelomic cavity is the key to the formation of the segmented structure of the body, and is the structure that determines the vertebrate forni. The coclomic cavity is present in a well defined state from the Alnelida tuwards, so that in ainielides it is performing the functions for which a coelom was evolved. It is, however, necessary to observe the conditions prevailing anon)g still lower forms to see why a separate cavity was formed in animals, which became the means of raising them in the scale of organisation, and ultimately leading to the evolution of the vertebrate animal. I therefore propose to trace the steps in evolution by which, I presume, the coelomic cavity originated, and then show how it or its modifications have been the basis on which the whole vertebrate structure of animals is founded. In demonstrating this the various steps or stages in the evolution of the vertebrate endoskeleton will be indicated clearly. In the evolution of the vertebrate endoskeleton five distinct stages occur. These are: 1. The Hydroskeleton. 2. The Hydrostatic skeleton. 3. The Sclerotome skeleton. 4. The Neural skeleton. (Notochord.) 5. The Neuro-miuscular skeleton. (Vertebrate skeleton.) 120 J1V. B. Pritmrose 1. THE HYDROSKELETON Ill the large and loNvest group of the Metazoa, namely the Coelenterata, Awe, for the first time fiuid animals possessing three distinct kinds of tissues. The outer of these is the ectoderm, and is more or less epithelial. This is a protective and receptive layer of cells which are modified in many ways to perform both of these functions. IIn all animals those structures which keep the animal informed about its surroundings, and which control its reactions to them, are derived from this layer. The middle layer or mesoglea is a some- what structureless tissue developed to very different degrees of organisation in different Coelenterates. It represents the mesoderm of higher animals in which it comes to form the bulkiest portion of the animal tissues. The inner layer is the endoderm. It is the layer that nourishes the animal, and it lines a cavity called the entcron. The cells of this layer are principally digestive, )but some are arranged for entrapping and killing the animaleulae pl)o0 which these animals feed, and which are drawl into the enteric cavity with the circulating water. The enteric cavity is the digestive and respiratory organ of the animal and it only possesses one opening which serves both for mouth and anus. Around this opening there are muscle fibres arranged to act as a sphincter. This cavity is always kept filled with water, which however, is being constantly changed by the activities of the animal for metabolic purposes. Ill addition to the physiological characters of this cavity and its contained fluid, we recognise a fact of great morphological importance. This is that the water contained in the enteric cavity has the essential characters of a skeleton. Thus it gives form to the animal, and maintains this form. Being an incompressible fluid it gives the animal's muscle fibres a resisting body to act upon, and so allows of alterations of shape, and in free swimming forms allows of )roplulsion through the water. When the water, which is the skeleton of the animal, is removed from its enteric cavity, the supporting structure is gone and the animal loses its shape just as when it is dead. This is the hydroskeleton and is the first stage in the evolution of the vertebrate endoskeleton. Fig. 1 shows the conditions present in a hydra polyp where we find the three layers of tissues mentioned above, and the enteric cavity. Where the oral tentacles are hollow the enterie cavity extends into them so that they likewise derive their support from the water in the enteric cavity, and their movements depend upon its presence. Fig. 2 shows the disposition of parts in a medusa form where the umbrella or under surface of the animal contracts upon the water in contact with it as a means of propulsion. Fig. 3 shows the arrangement of parts in a sea-anemone. The same three layers are present and the enteric cavity; also hollow or solid tentacles. The mouth opening is present and leads down into the enteric cavity through a tubular invagination called the oesophagus. The enteric cavity is incompletely subdivided by a number of longitudinal septa passing from the body wall towards the centre ending in a free margin. Certain of these septa, however, The Evolution of the Vertebrate Endos1keleton 121 attach themselves to the oesoplhagus forming incomplete compartments in the enteric cavity. These cavities extend into the hollow tentacles. On these septa are placed the reproductive glands which discharge their cells into the enteric cavity. These animals also depend 11po)0 the water contained in the enteric cavity for the maintenance of their form and for providing a means for their muscles to produce movements, so that here again we find the hydroskeleton in the enteric cavity. This type of skeleton is evidently efficient in its )rinciple, niamcly, that water is dense and incompressible, but the structure of the animal limits the efficiency of the hydroskeleton by allowing the water to escape easily, which is necessary, as frequent change of water is required by the animal bor metabolic )lrposes. The enteric cavity here contains the skeleton, digestive, respiratory, excretory and reproductive systems. These arc all represented iii a very simple FYr. I Figr. 2 Fig. 3 Fi(g. 1. 1)iagram of Ily(Ira polYp showing enteric cavity which contains the hydroskeletoii. Fi(g. 2. J)iagtran of the same )arts in a niedusa. Fi(r. 3. SectiOl through the oesop)hagus of a sea-anemione showing enterie spaces aisd inesenteries. After Ship)ley and MacBride. form, while in higher animals these systems all become differentiated and specialisedi. In concluding the remarks on the hydroskeleton of coelenterate animals, I would again point out that the skeleton is the water contained in the body of the animal. It is this water that maintains the form of the animal, and it is llpOl) this water that the muscles ofthe animal act in order to effect movements of parts of the body on one another, or the propulsion of the animal as a whole. The skeleton being the water within the body of the animal, and the body cavity never being shut off from the water in which the animal lives, it is not very efficient as a skeleton, and dooms the whole class of coelenterate animals to an aquatic existence. So soon as the animal is removed from the water its skeleton is lost, and it has then no means of maintaining its form, and still less has it anv means of continuing its existence. 122 W. B. Primrose 2. THE HYDROSTATIC SKELETON In the next stage in the evolution of the vertebrate endoskeletoni the chief disadvantages of the hydroskeleton are overcome by confining the water which forms the skeleton in an enclosed space. This is effected by the complete separation of the enteric pouches from the enteric cavity. The arrangement of the mesenteric septa in relation to the oesophagus or stomodaeum in actinian forms strongly suggests how this could be accomplished as in fig. 3. The continuance of the process by which enteric pouches are formed would result in the complete separation of these pouches from the enteric cavity. This would form a new set of enclosed spaces situated between and separating the enteric cavity from the body wall. I do not propose to do more than suggest such a method of producing these perienteric spaces, for we find a different method obtaining in some other animals. In coelomate animals we are presented with this condition as an accomplished fact. It is this condition that makes animals coelomate, and separates these entirely from the whole group of Coelenterates. It is therefore a condition of the highest importance in the evolutionary history of animals. These perienteric cavities are present in some form in all animals higher in the scale of organisation than Coelenterates, and the reproductive cells are always produced in their walls, just as they are in the sea-anemone. There may be pores in the body wall of the actinozoon, and also in hollow tentacles, by means of which the reproductive cells can escape to the exterior of the body. In the higher forms more efficient means are adopted for the same purpose. We have now to deal with coelomate animals which are characterised by having a perienteric space separating the body wall from the enteric cavity which now forms the gut wall. The lowest annelid shares this common character with the highest vertebrate. This perienteric space is the coelomic cavity, and this cavity is of the first importance as a skeletal structure, or, more accurately, as a space containing the- skeleton, and it with its derivatives will be described from this point of view.
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