The evolution of a secondary body cavity appears to have been of major and fundamental importance in metazoan evolution in that it seems an essential prerequisite to the development of greater size and complexity. However, the study of the evolution of the secondary body cavity must, by the nature of the structure, rely solely on indirect evidence since direct evidence from palaeontology is not available. This indirect evidence is chiefly from the study of the embryology of present-day metazoans and it is in the interpretation of this evidence that widely conflicting ideas and theories have arisen. Another area of disagreement between zoologists has been over the very nature of the various body cavities found in the various animal groups. The evolution of the coelom has recently been extensively discussed (Clark, 1964).

    The coelom is defined as a liquid-filled cavity in the mesoderm; it develops in, and is surrounded by, the embryonic mesoderm. In some groups, by contrast, the body cavity is formed by a persisting of the blastocoel. This cavity is a false coelom or pseudo-coelom and is simply a space between the digestive tract and that part of the body wall not bounded by mesoderm. It is obvious that the only reliable way to characterise the exact nature of the body cavity of a particular animal is to study its embryology. Although our knowledge of invertebrate embryology is still incomplete, enough information is available to 'facilitate arrangement of the various groups of the Bilateria according to the nature of their body cavities.

    There are three major groups: 

(l) Acoelomate: The region between the digestive tract and the epidermis is completely filled with mesenchyme and muscle fibres, with no body space present (Fig. 6, A). Phyla Platyhelminthes and Nemertini. 

(2) Pseudocoelomate: The body space is a pseudocoelom (Fig. 6, B). Phyla Acanthocephala, Aschelminthes (Rotifera, Gastrotricha, Kinorhyncha, Nematoda, Nematomorpha) and Entoprocta. 

(3) Coelomate: The body space is a true coelom (Fig. 6, C). The remaining phyla of the Bilateria, Phyla Annelida, Arthropoda, Mollusca, Priapuloidea, Bryozoa, Phoronida, Brachiopoda, Chaetognatha, Echinodermata, Hemichordata, Chordata. The coelomate group can be again divided according to the type of coelom present, each type determined by the manner of origin of the entomesoderm and coelomic cavity. According to embryological studies, there are three basic ways in which the entomesoderm and the coelom can arise (Hyman, 1951 ):

(a) Schizocoel: 

    The coelom arises from a split in the mesodermic bands, plates or masses. It occurs in teloblastic mesoderm formation in spiral cleavage, in the derived mesoderm formation from cells around the blastopore as in most arthropods, or in lamellar mesoderm formation where the mesoderm originates from a mesenchymal origin. The groups in which the coelom is believed to have arisen as a schizocoel include most of the Protostomia. 

(b) Enterocoel: 

    The coelom is thought to arise from the cavities in mesodermal sacs which evaginate from the archenteron and which expand until they touch the gut and body walls. This method of coelom production is seen in the Deuterostomia or chordate line and in the Brachiopoda. 

(c) Mesenchymal: 

This is an unusual method of coelom production seen only in the Phoronida; it can be regarded as an aberrant form of schizocoel. In this method the mesenchyme rearranges itself to enclose a space, thus forming a coelom. 

   That the evolution is not questioned; the great increase in size and diversity in way of life in the coelomate groups of animals compared with the acoelomate and pseudocoelomate groups must make this obvious. However the mechanism of that evolution and the question as to whether it occurred only once or many times by different routes has been the subject of many theories. There are four basic theories of the evolution of the coelom, the other theories being simple variants of these basic ideas. A lengthy discussion of these is given by Clark ( 1964) and the following description is only a brief summary of each theory.




(a) Enterocoel Theory 

This theory accepts the origin of the coelom from gastric pouches as primitive and suggests that it indicates the phylogenetic origin of the coelom. Sedgwick (18 84) suggests that the gastric pouches of certain coelenterates, principally Anthozoa, became separated from the main gastric cavity to form coelomic pouches (Fig. 7). The theory is widely followed and detailed discussions of it are put forward by Hartman (1963) and Remane (1963). The chief arguments against this theory are that gastric pouches only occur in more advanced coelenterates not very suitable for ancestral types; that the sealing-off of the gastric pouches defeats the object for which they were formed; and that most supporters of this theory tend to associate the evolution of the coelom with that of metameric segmentation, an association imposing severe restrictions on the theory. 

(b) Gonocoel Theory 

This is perhaps the most widely held of the theories of the origin of the coelom and is based on the common association between the gonads and the coelomic epithelium; it regards the coelom as the cavity of an expanded gonad. The theory was first enunciated by Bergh in 1885 (cit. Clark, 1964) who compared the segmented coelom of annelids with the linear series of gonads of flatworms and nemerteans (Fig. 8). One of the main arguments against the theory, like that of the enterocoel theory, is that it closely links the origin of the coelom with that of metameric segmentation. The main difficulty is to account for unsegmented coelomates. Another argument against the theory is that it regards as separate processes the formation of endomesoderm from the inward migration of gonadial cells and the formation of the coelom by cavitation of the gonads after the release of gametes. This is contrary to the facts displayed by embryological studies. 


(c) Nephrocoel Theory 

The theory proposes that the coelom originated as an expanded nephridium. It is not a theory held seriously by many, the chief opposing arguments being that protonephridia have been described in coelomates, and that some coelomate groups, such as the echinoderms, do not have excretory organs. 

(d) Schizocoel Theory 

This theory has very little mention in the literature. The coelom is regarded as a new formation from the mesoderm, which, by this theory, has a mesenchymal origin, and is not related to gonads or entodermal pouches and has no antecedents in the lower forms. None of the above theories is entirely satisfactory for three main reasons. Firstly, they largely ignore the intermediate stages passed through in the course of evolution and the advantages which these conferred on their developers. That such intermediate stages were advantageous is mandatory for their evolution and their adaptive significance should be considered. Secondly, the relationship between the evolution of the coelom and the evolution of metameric segmentation should have been elucidated and if their connection is necessary then an explanation of unsegmented coelomates should be included. Thirdly, there is no clear statement concerning the exact nature of a coelom and explaining which cavities should  be regarded as coelomic and which not, in specific cases. A discussion of evolution of the coelom including these topics and an account of the significance and functions of the coelom is presented by Clark (1964); the following discussion is based largely on this work.




    In studying the evolution of the coelom and its significance in metazoan evolution, the key evidence is the likely function of the coelom that made it of such evolutionary advantage. Of all the various functions prescribed for the coelom the one which best fits this requirement is a purely mechanical one: the coelom provided the animal with a hydrostatic skeleton. Strong evidence for this is found in the fact that in the groups of animals where it does not serve this function the coelom is severely reduced.

    In the early stages of evolution of the Bilateria the body wall musculature probably consisted of only a few contractile elements permitting only feeble movements. As the body size and musculature increased and animals abandoned a free swimming existence to live on the substratum, locomotion by ciliary activity became less efficient. Even with a more advanced body muscular system made up of longitudinal and circular layers permitting reversible changes in body shape, the limitations imposed on movement by a solid body are severe, as is illustrated by the platyhelminthes. These limitations are certainly too severe to permit strong burrowing. The limitations imposed on the circular and longitudinal muscle systems would be lifted if, instead of a solid body, a true fluid skeleton were evolved to permit strong, antagonistic contractions of the circular and longitudinal muscles and a controlled local change of body size. For this the morphological nature of the fluid filled cavity is of no importance so long as it can serve this mechanical function. Indeed, the embryological and morphological evidence gained from modern animals, to say nothing of the number of totally irreconcilable theories concerning the evolution of the coelom, suggest that a fluid skeleton was evolved independently several times. Moreover, there is no evidence to suppose that the secondary body cavity is homologous throughout the animal kingdom. In fact the contrary is indicated, that is, that the coelom is polyphyletic in origin.