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On the origin of the biclitellate condition in lumbricid earthworms


On the origin of the biclitellate condition in lumbricid earthworms



Ann And Mag Nat Hist 9(104): 577-581



Two clitella on a single individual, always asymmetrically developed, have been found only in lumbricid earthworms and only in association with metameric abnormalities involving monolateral increase or decrease in number of adult segments. The conditions can have resulted from (1) fragmentation, on one side of the embryo, of an appropriate no. of mesoblastic somites in such a way that each portion retains developmental potentialities of the parent block. The halving of somites, apparently required by individuals of the "relictus" sort, is not always adequate, in the present cases, at least some trisection being required. (2) abortion, on one side of the embryo, of an appropriate no. of somites; (3) fragmentation and abortion in the same embryo. Inherent in the explanation of the biclitellate and relictus conditions are certain morpho-genetic principles that can be stated as follows: epidermal metamerism is determined by the mesoblast. Determination for late ontogenetic development of the clitellum is a function of original numerical sequence (axial position) of embryonic somites. The presumptive clitellar somite retains its potentiality regardless of subsequent displacements and later is able to induce differentiation of some cocoon-secreting glandular tissue in a non-clitellar mesoblastic block with which it becomes united. In absence of such an induction, an adult segment, even though in the proper position, does not become clitellar. Determination for gonad differentiation also is a function of original axial position of the embryonic somites. Testes and ovaries mature and induce development of organs such as gonoducts, seminal vesicles and ovaries regardless of considerable axial displacement. Data from aberrant individuals may provide important evidence as to morphogenesis in metamerically segmented animals in which experimental modification of development has not been attempted.

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