The zygospore becomes surrounded with its own wall, consisting finally of three layers, the outer of which is furnished with spicular prominences of various forms. In Zygnemaceae there is no dissolution of the filaments, but the whole contents of one cell pass over by means of a conjugation-tube into the cavity of a cell of a neighbouring filament, where the zygospore is formed by the fusion of the two FIG.
In addition to the spicular skeleton an organic horny skeleton is frequently present, either in the form of a horny external investment (Cornularia), or an internal axis (Gorgonia), or it may form a matrix in which spicules are imbedded (Keroeides, Melitodes).
The silica was derived from the tests of radiolaria and the spicular skeletons of sponges.
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