VI - The digestive system of Amphioxus (Branchiostoma) Lanceolatus

Abstract

In view of the interest of Amphioxus as a primitive Chordate type, and its wide use in zoological teaching, it is remarkable that so little should be known of the structure and physiology of its digestive system. The early treatise of Delage and Hérouard (1898) gives little information concerning the mid-gut and the so-called “liver” beyond referring to their green colour, which is ascribed without further elucidation to the presence of secretory granules, while the hind-gut “ne présente rien de particular”. Pietschmann (1929), in his recent excellent account of the Cephalochorda, can give little further information. The epithelium of the “ liver” and “ stomach” are described as composed of ciliated cells with granulated cytoplasm, but no suggestion of regional differentiation in the various parts of these organs is given. Of the function of the hinder region of the alimentary system nothing can be said beyond a reference to the spiral movement imparted to the food by the ilio-colon ring, while Hammar’s statement, based on an embryological study (1898), that the “liver” is homologous with the liver of the higher Chordata, is accepted without question. The essentially physiological monograph of Franz (1927 b )is equally uninformative. It is, then, evident that this alimentary system demands a complete investigation both from the structural and functional points of view, and it has, in fact, been impossible to deal with all the problems which have suggested themselves. In its present form the work provides a description of the ciliary mechanisms of the post-pharyngeal regions of the gut, together with some account of the cytology of the epithelium and of the digestive enzymes secreted by it, and discusses in the light of this description the probable mode of operation of the mechanisms and the function of the various parts of the system; in conclusion, the homology of the “liver” is discussed, and a new interpretation of this organ suggested. It is hoped to undertake in the near future a comparative study of the cytology of the alimentary canal of the lower Chordata, and the cytological portion of the present work is therefore not to be regarded as exhaustive.