Studies on neothyris lenticularis (Deshayes).

Abstract

Neothyris lenticularis (Deshayes), the only living representative of its genus, is a member of the Order Telotremata within the Sub-Class Pygocaulia. Its taxonomic position has been discussed by Thomson (1915). As a result of recent investigations upon the embryology of Terebratella inconspicua, Percival (1944) found Thomson’s Gastrocaulia and Pygocaulia to be inadequate as sub-classes and suggested that it may be necessary to place them in two separate phyla. Percival also reviewed the anatomical features which he considered were convergent in the two sub-classes, and the structure of the shell, concluding that the "Brachiopoda consist of two very sharply differentiated groups from the standpoints of embryology and physiology."

It is from this latter point of view that the present study has been undertaken, for it is considered that by a simultaneous study of form and function in as many families of brachiopods as possible, it may be determined which structures are homologous and which are modifications for a particular mode of life. Yonge (1939) from his studies on Lamellibranchs, is of the opinion that an adequate knowledge of the mechanisms of respiration, feeding and digestion provides the key to the interpretation of structure in the members of a class. The only account of the physiology of feeding in brachiopods is that of Orton (1914) on Crania. His account does not include a description of the processes of digestion, a subject on which only scattered references are available in the literature of brachiopods. This study aims at providing detailed descriptions of the ciliary feeding mechanism and of the structure and functions of the gut of N. lenticularis which are compared, as far as possible, with similar structures and processes in other brachiopods, particularly those of the Sub-Class Gastrocaulia. In addition, certain aspects of the anatomy - the blood and nervous system, which have been the subject of much controversial literature, - have been investigated. As specimens of Terebratalla sanguinea and T. inconspicua were available, the physiological processes and structures of these forms have been compared with those of N. lenticularis.

It will be noted that Percival’s recommendation (loc. cit.) that the nomenclature of the valves be reversed has been adopted, that valve containing the pedicle being dorsal, the brachial valve ventral. [Introduction]