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Connective Tissues and Body Wall Structure, of the Polychaete Polyphysia Crassa {Lipobranchius Jeffreysii) and Their Significance

The polychaete Polyphysia crassa (Oersted) is unusual in possessing a well-developed body-wall connective-tissue layer which exceeds the combined thickness of the circular and longitudinal muscles. Both collagenous and elastic fibres are present in this layer. The collagen is organized as a loose three-dimensional lattice allowing longitudinal, circumferential or radial distension of the body wall and, as in other soft-bodied invertebrates, serves the functions of providing a base on which the muscles can act and of imposing limits to the extensibility of the system. The elastic fibres are organized as an apparently randomly oriented meshwork of stout fibres around the circular muscle blocks and are attached to both the circular and longitudinal muscles. Columns of elastic fibres extend radially from the supramuscular coarse meshwork through the ‘holes’ in the collagen lattice to the epidermal basement membrane. As the elastic columns extend outwards the fibres become finer and more numerous and flute out to give a wide area attachment to the epidermal basement membrane. The radial elastic columns are cross-linked by tangential elastic fibres. Although at any point the main collagen and elasticfibre bundles are oriented at right angles to one another, collagen fibres are invariably associated with the elastic fibres and the two fibre types form a single functionally inte-grated system. Polyphysia lives in flocculent, sublittoral muds and the burrowing mechanism employed involves a pronounced direct peristaltic wave of simultaneous circular and longitudinal muscle contraction which necessitates considerable radial thickening of the body wall. The functions of the elastic fibres appear to be to oppose the radial distension of the body wall which the collagen lattice permits and to control the folding of the cuticle and epidermis and the return of the collagen system after the passage of a peristaltic wave.