47 research outputs found
A hexamer origin of the echinoderms' five rays
Of the major deuterostome groups, the echinoderms with their multiple forms
and complex development are arguably the most mysterious. Although larval
echinoderms are bilaterally symmetric, the adult body seems to abandon the
larval body plan and to develop independently a new structure with different
symmetries. The prevalent pentamer structure, the asymmetry of Loven's rule and
the variable location of the periproct and madrepore present enormous
difficulties in homologizing structures across the major clades, despite the
excellent fossil record. This irregularity in body forms seems to place
echinoderms outside the other deuterostomes. Here I propose that the
predominant five-ray structure is derived from a hexamer structure that is
grounded directly in the structure of the bilaterally symmetric larva. This
hypothesis implies that the adult echinoderm body can be derived directly from
the larval bilateral symmetry and thus firmly ranks even the adult echinoderms
among the bilaterians. In order to test the hypothesis rigorously, a model is
developed in which one ray is missing between rays IV-V (Loven's schema) or
rays C-D (Carpenter's schema). The model is used to make predictions, which are
tested and verified for the process of metamorphosis and for the morphology of
recent and fossil forms. The theory provides fundamental insight into the
M-plane and the Ubisch', Loven's and Carpenter's planes and generalizes them
for all echinoderms. The theory also makes robust predictions about the
evolution of the pentamer structure and its developmental basis. *** including
corrections (see footnotes) ***Comment: 10 pages, 6 figure
A bibliography of parasites and diseases of marine and freshwater fishes of India
With the increasing demand for fish as human food, aquaculture both in freshwater
and salt water is rapidly developing over the world. In the developing countries,
fishes are being raised as food. In many countries fish farming is a very important
economic activity. The most recent branch, mariculture, has shown advances in
raising fishes in brackish, estuarine and bay waters, in which marine, anadromous and
catadromous fishes have successfully been grown and maintained