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Summary.

  • 1
    In certain species of Tetraxonida the period of adult life may, in some cases, be subdivided into as many as three distinct growth stages, each characterized by some evident change in the form of the spicules, coupled with changes in the arrangement of the skeleton, or even with differences in the gross morphology.
  • 2
    A tangential skeleton is a primitive character, and species in which this persists throughout adult life may be regarded as having undergone little specialization.
  • 3
    Extrusion of spicules takes place either continuously or at definite periods in the life cycle. This has been assumed by previous authors, but without definite reason for so doing, and the proof now forthcoming enables us to explain the absence of particular categories of spicules from certain individuals of a species, the disorganization of the skeleton SO often seen, and the differences in the arrangement of the dermal skeleton in different individuals of the same species.
  • 4
    In the identification of species not only must the normal fluctuating variations and those due to ecological, geographical, and environmental factors be considered, but also those due to age.
  • 5
    The order in which different forms of spicules appear in the life cycle indicates the order of their appearance in the evolution of the species, anti probably, although this yet remains to be proved, when reduction of the skeleton takes place the last spicules to appear are the first to disappear.