No abstract

Gravimetric, radiotracer, and indicator methods currently available for estimating assimilation efficiencies, have been reviewed and their associated limitations have been discussed. It was concluded that the basic assumption implicit to gravimetric and indicator techniques, i.e. that all material contained within the faeces is derived from the food, does not generally hold. Radiotracer techniques are not based on this assumption but are time consuming. Consequently a new radiotracer technique analogous to indicator methods has been developed. In this technique the concentration of a non-absorbed indicator is expressed in terms of a radiotracer, C, which can be absorbed but which, at least initially, is only present in the food, rather than expressing it in terms of dry weight.Cr has been used as the nonabsorbed indicator.Use of these two isotopes in conjunction not only enables a distinction to be made between faecal material derived from food, and that derived from metabolic secretions but also facilitates estimation of assimilation efficiences fromsmall samples of faeces only. The new technique requires simply, measurement of the ratioC:Cr in samples of both food and faeces.The applicability of conditions necessary for operation of the new technique has been tested on two species of freshwater gastropod, one feeding on epilithic algae, the other on bacteria, and its effectiveness has been tested by reference to results obtained from another, more conventional method involving C only.

Epidermal Langerhans cells bear surface receptors which implicate them as immunocompetent cells and they are now felt to play an important role both in delayed hypersensitivity and in skin allograft reactions. To determine the relationship between Langerhans cell availability and certain immunologic phenomena, surface densities were determined by ATP-ase and gold uptake in 3 rodent species: guinea pig, hamster, and mouse. Surface densities in epidermal specimens from the ear, back, foot pad, and buccal mucosa varied between 600 and 1500 cells/mm2. Significantly fewer cells were found in the hamster cheek pouch (130 cells/mm2) and in the mouse tail (110 cells/mm2 for C57BL/6J; 260 cells/mm2 for BALB/c nu/nu). Langerhans cells were absent from the central port;on of the cornea in all 3 species. Decreased Langerhans cell surface density may contribute to immunologic privilege as has been observed for the cornea and hamster cheek pouch and to the unusual allograft characteristics of mouse tail skin.

No abstract

The sibling marine snails Littorina obtusata (L.) and Littorina mariae Sacchi & Rastelli are sympatrically distributed and the shells of both species are subject to similar breaking forces by predatory crabs. Nevertheless, the two species exhibit rather different growth and defence strategies. To determine growth patterns, we measured changes in five morphological variables with increasing shell length: body whorl thickness at the point of crushing force application, shell height (related to globosity), shell mass, body mass, and apertural lip thickness. We also measured ontogenetic changes in the ability to withstand shell crushing. For most morphological variables, L. mariae showed uniformly allometric growth of juveniles into adults. In contrast, L. obtusata usually exhibited a distinct change in growth pattern upon reaching maturity. As adults, L. mariae showed a more sustained increase in overall shell mass and in body whorl thickness (defence against crushing attacks) and also had proportionally thicker apertural lips (defence against peeling attacks). Littorina obtusata, however, grew to a larger size and their shells could accommodate larger bodies at all sizes. Furthermore, the strength of L. obtusata shells increased faster than could be accounted for by either overall shell mass or thickness at the point of force application, suggesting strengthening by other means such as changes in shell microstructure or shape (other than globosity). These results illustrate the viability of two contrasting antipredator strategies, despite a highly similar phylogenetic history and selective regime.