This study of the development of a rhynchonellid brachiopod has revealed marked differences from that of Terebratella inconspicua (Percival, 1944). These principally concern the origin of the mesoderm and of the lophophore. The origin of theright and left mesoblastic sacs in Tegulorhynchia and in Terebratella depends on the stage at which the blastoporal groove exerts its influence, earlier in the former, leading directly to their formation, later in the latter, leading indirectly to their formation. The subsequent division into anterior and posterior sacs is similar in both genera. The lophophore of Tegulorhynchia is the product of the anterior lobe, and considerable centripetal movement occurs during the sinking of the centre from which the brachial fold (or buccal lip) is derived. The process of formation of the lophophore in Terebratella can be regarded as a much shortened version of that which occurs in Tegulorhynchia. In the latter genus the origin of the brachial fold is elaborate and protracted, while in the former the brachial fold does not appear until 8 pairs of cirri have been formed.

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