Functional morphology of the Hippuritidae

The hippuritid shell consists of a small operculiform left valve capping a larger cylindrico-conical valve. There are three shell layers: (1) An outer calcitic fibrillar prismatic layer. (2) A crossed-lamellar aragonitic middle layer. (3) A complex crossed-lamellar aragonitic innet layer. There are also traces of myostracal prismatic aragonite which mark former adductor positions, the attachment site of the body mass, which hung largely from within the left valve, and the pallial curtain of the right valve alone. It =ms that water was drawn through the pores on the outer surface of the left valve, into the radial canals which lie within its outer layer. It then flowed outwards from the canal apertures, passing over the broad and radially crenulated right mantle margin, whereon food particles were trapped. The particles were sorted and passed inwards anterieventrally onto the ctenidia and/or palps, which carried them to the mouth. Faeces were ejected via the dorsalmost of two oscules in the left valve, and pseudofaeces via the ventralmost oscule. The form of the adductor muscles and their orientations upon myophores projecting from the left valve suggest immovable valves only separated by a minute gape. These modifications complemented the atrophy of the normal bivalve internal feeding and respiratory current system. La coquille des Hippuritidae comprend we valve gauche operculiforme et une valve droite cylindro-conique. Le test est formé de trois couches: 1. Une couche externe constituée de fibres prismatiques de calcite; 2. Une couche moyenne aragonitique à structure lamellaire entrecroisée; 3. Une couche interne aragonitique à structure lamellaire entrecroisée com-plexe. Les zones d'insertion des muscles adducteurs, de la masse viscérale et des muscles palléaux, montrent en outre des traces de primes myostracaux d'aragonite. Les courants d'eau inhalants pénéraient par les pores de la valve gauche et cheminaient dans les canaux radiaux qui parcourent la couche externe, pour déboucher à la cornissure des deux valves; où les particules alimentaires étaient retenues. Les particules étaient triées et acheminées vers la région antéro-ventrale de la cavité générale en direction des branchieslou des palpes et de h bouche. Les fèces étaient rejetés par l'oscule dorsal de la valve gauche, les pseudo-faxs par l'oscule ventral. La forme courte des muscles adducteurs et leur orientation sur les apophyses myophores de la valve gauche suggère que les valves étaient immobiles et ne laissaient entre elles qu'un faible intervalle. Ces modifications vont de pair avec I'atrophie du système digestif et respiratoire.     

The sensory basis of trail following in some lepidopterous larvae: contact chemoreception

ABSTRACT. . Caterpillars of the genus Malacosoma follow trails of the chemical 5-beta-cholestane-3,24-dione, but nothing is known of how they perceive this compound, or more generally about the sensory basis of trail following in caterpillars. By selective ablations of chemosensory organs we show that, in Malacosoma , the trail chemical is perceived by the maxillary palpi. In another lepidopteran species, Yponomeuta cagnagellus , the palpi are needed to discriminate their own trails from a trail of Malacosoma. Malacosoma larvae also lose their specificity for conspecific trails when their palpi are ablated. Volatile cues evidently do not play a role in trail-following behaviour, since neither Malacosoma nor Yponomeuta can orient on a trail covered with fine nylon mesh. These data indicate that for Malacosoma , and probably also for Yponomeuta , contact chemoreception mediated by the maxillary palpi is the primary mode of pheromone perception. The evolution of receptor sensitivity to trail chemicals in caterpillars is discussed.     

Trilobite size-frequency distributions, recognition of instars, and phyletic size changes

Size-frequency analysis of over 5,000 Ordovician trilobites from the Teretiusculus Shales of the Builth inlier, central Wales, has revealed size distributions with counter intuitive shapes. Not only do most species show normal or slightly skewed distributions, despite the preponderance of moults, but there is no evidence of instar peaks. Such features can, however, be explained by reference to steady-state population structures of Recent marine arthropods, in which small individuals often form only a minor proportion of the post-larval population structure. Trilobite steady-state population structures would have differed in detail from species to species, but certain distribution shapes may have been characteristic of particular environments. These findings necessitate a reappraisal of previous work on trilobite size-frequency distributions, survivorship and recognition of instars. The Builth data also show the first clear evidence of phyletic size increase and parallel size changes in trilobites. ▭ Trilobites, size-frequency distributions, steady-state populations, instars, phyletic size changes.     

Rats,pigs and men: disturbance and diversity in the New Guinea highlands

A trapping study of four species of Rattus and three of Melomys in rainforest and grassland habitats revealed differences in altitudinal distribution and in both gross and subtle habitat selection between species. The species are ranked on a scale of increasing opportunism on the basis of various demographic characteristics (e.g. reproductive rate, life span and population density). Species judged to be more opportunistic occur in habitats subject to more intense disturbance from pigs and humans. It is argued that changes in the pattern of species coexistence have followed overt human impact in the New Guinea highlands.     

Changes in a forest bird community during a period of fire and drought near Bega,New South Wales

The bird community of a forested 13 ha plot was studied for 3 years, during which the plot was burnt by wildfire and was severely drought-affected. The combined effects of fire and drought were surprisingly limited. Similar numbers of species were recorded in each year and differences in species composition involved only rare visitors to the plot. Colour-banding indicated population declines in at least six resident species. The greatest decline was that of the Brown Thornbill (Acanthiza pusilla), from 20 pairs to 10 pairs. In general, insectivorous birds were most abundant and widespread on the plot during the first year, before the fire. In contrast, nomadic birds feeding on flowers or fruits were most abundant in the second year, when there was a prolific flowering and fruiting of several important food plants. After the fire, at least ten bird species avoided the burnt ridges and became more concentrated in the unburnt gullies. Most were species associated with dense shrub thickets, the layer of vegetation most affected by the fire. It seems that gullies are major refuge areas in these forests and are the key to the resilience of the bird community to fire and drought.     

The Structure and Development of the Reproductive Organs and Carposporophyte in two British Species of Gelidium

The structure and development of the reproductive organs andcarposporophyte are described for two British species of Gelidium,G. latifolium (Grev.) Born. & Thur. and G. pulchellum (Turn.)Kütz. The gonimoblast does not develop directly from theunchanged carpogonium, as was thought by Kylin, but a swollenmultinucleate cell of irregular outline is formed first, eitherfrom the carpogonium alone, or by the fusion of the carpogoniumand certain neighbouring cells; the gonimoblast develops fromlong, non-septate processes of this multi-nucleate cell. Thesignificance of the fusion of the carpogonium and certain vegetativecells is discussed in relation to the various definitions ofthe auxiliary cell which have been proposed. It is concludedthat an auxiliary cell does not occur in Gelidium.