Hinge teeth

Abstract: Growth processes and patterns set functional limits on the adaptive range of organisms that have evolved a particular “Bauplan.” The design of the elastic ligament has played a large part in determining the scope of the evolutionary radiation of the Arcoida. Tensional lamellar and compressional fibrous materials of this ligament, which appear to be similar in their elastic properties to those of other bivalves, are not segregated in positions where they could best perform their different mechanical functions. The growth of this unspecialized ligament exhibits strong positive allometry, with respect to shell size, in many fossil and living arcoids. A simple mechanical model shows that this allometry is essential if the strength of the ligament is to keep up with the weight of the animal, during ontogeny. Inherent disadvantages of the allometry include increasing dorsal breakage of the ligament itself, interference with the function of the hinge teeth and reduction of the closing moment exerted by the adductor muscles. Direct measurements of living arcoids show that allometric growth just maintains a linear relationship between ligament strength and animal volume. Major differences in ligament strength are related to shell growth patterns and substantially different environmental adaptations. Ligament strength is highly variable within individual populations; between populations, it is correlated with shell thickness and local environmental conditions. Most arcoids are adapted for shallow-burrowing or epifaunal modes of life in physically unstable environments, where they are frequently disturbed. The large, muscular foot enables these mobile animals to regain suitable living positions after such disturbances. The weak ligament has limited the potential specialization of the arcoids for either deeper burrowing or permanent epibyssal attachment, also requiring the retention of the foot in epifaunal forms. This ligament has made diverse adaptations possible, while preventing extreme specialization, except in isolated taxa. As such, it is largely responsible for the evolutionary reversals documented by Stanley (1972). This is konstrukionsmorphologie No. 46.

Abstract: A uniaxial orthopaedic hinge (1000, 2000) has a substantially circular body forming a recessed housing (1022, 2022). The upper surface of the housing (1022, 2022) is furnished with a concentric arcuate circular slot (1038, 2038) disposed inwardly from the periphery. The inner surface (2040) is provided with equally spaced radial teeth (1044, 2044). A back plate (1006, 2006) has an extension into a stub arm (1004, 2004) for attaching to the arm of an orthosis and is fixed to the recessed housing (1022, 2022). A locking plate (1008, 2008) also has an extension into a stub arm (1002, 2002) and is disposed in a parallel manner between the recessed housing (1022, 2022) and the back plate (1006, 2006). Adjustable locking means are in the form of quadrants (1050, 1052, 2050, 2052) each having a pusher (1066, 1068, 2066, 2068) which extends through the arcuate slot (1038, 2038) in the recessed housing (1022, 2022) and toothed locking faces (1042, 2042) on either side of a base portion (1054, 1056, 2054, 2056), the teeth being adapted so as to engage with those on the inner aspect (2040) of the recessed housing (1022, 2022). The quadrants (1050, 1052, 2050, 2052) are each provided with a compression spring (1074, 1076, 2074, 2068) in the base which sustains the engaged condition until the pusher (1066, 1068, 2066, 2068) is depressed after which the quadrants (1050, 1052, 2050,2052) may be moved along the slot (1038, 2038) to a new position. The locking plate (1008, 2008) is provided with an flange abutment stop (1088, 2088) disposed in such a manner that it always lies under the slot (1038, 2038) in the hinge body and adapted to co-operate with the quadrants (1050, 1052, 2050, 2052) so that the hinge (1000, 2000) may be locked in a selected position or allowed any desired range of angular motion which is a multiple of the angular increments of the hinge teeth angle.

Abstract: The anatomies of two species of Neoleptonidae are described. Clearly heterodont with cardinal and lateral hinge teeth, the shell also has an internal, parivincular ligament. Shell form suggests a shallow burrowing mode of life in coarse gravels in coastal waters. The labial palps are small and the intestine short further suggesting deposit feeding in well-sorted gravels. A pronounced prodissoconch (II) with a marginal ridge argues for lecithotrophic development further facilitating re-colonization of a narrow niche. In most anatomical respects, the two species are simplified with conjoined inhalant and pedal apertures and few posterior sensory mantle papillae. The ctenidia comprise subequal demibranchs with the outer reduced and the inner modified for internally fertilized embryo brooding. The attachment of each embryo chord to the demibranch filaments is probably from secretions produced by basal glands developed on their abrofrontal surfaces. There may also be secondary external pallial broodi...

Abstract: Philobrya (Philobrya) munita was originally described as P. (Hochstetteria) munita Finlay 1930, but the features of the shell and ligament clearly ally it with the former subgenus. P. munita, unlike the majority of its closest relatives which are benthic, lives intertidally attached by a stout byssus to the holdfasts of algae on moderately exposed shores in New Zealand. P. munita is monomyarian though it possesses a shell which has the basic heteromyarian form with an inflated posterior slope (and correspondingly enlarged posterior adductor and posterior byssal retractor muscles) and a reduced concave anterior slope (with a reduced anterior byssal tetractor inserted on the posterior slope of each valve). The umbones are beak-like and there is a distinct byssal notch. Internally a septum or modified hinge plate is located under the opisthodetic alivincular ligament. There are no hinge teeth or denticles. The significance of the shell and ligament of P. munita is discussed in relation to the basic duplivincular ligament of other arcoids and the ligament of other bivalves. P. munita possesses a posterior inhalant aperture only. This is unusual since some other members of the Arcoida have both anterior and posterior inhalant streams. The posterior inhalant stream in P. munita results from the very great reduction of the anterior slope of the shell. The ciliary currents of the mantle cavity are described; the ctenidia are of type B(la) and there is a single cephalic eye located just anterior to the anterior filament of each ctenidium. The roof of the supra-branchial chamber above the inner demibranch possesses a hypobranchial gland. The labial palps are small and there is a labial caecum located within the inner lip of the mouth. The mantle comprises three folds, with the outer fold possessing a secondary fold. The outer mantle fold bears a row of pallial eyes; light reception is thus through the periostracum. The structure of the stomach has been reconstructed from serial sections and the basic histological features of the alimentary canal are described. The pericardium possesses a heart with a single ventricle and a pair of auricles which communicate with each other posteriorly. The kidney and gonads discharge via a common urino-genital aperture. P. munita incubates fertilised eggs within the infra-branchial chamber of the mantle cavity; incubation within the supra-branchial chamber being impossible with a filibranch ctenidium. P. munita is compared with other philobryids, other members of the Arcoida and with other bivalves. It is concluded that P. munita is a very unusual bivalve–the morphological eccentricities it possesses resulting from its primitive arcoid origins, the evolution of the heteromyarian form, and specialisations not seen in other bivalves.