Before continuing the description of the alimentary tract, there are two general questions which must first be discussed very shortly. So far we have only dealt with the first two parts, the buccal cavity and the pharynx, and the latter name has such a well-defined significance that it is easy to state the boundaries of that part of the canal to which it should be applied, and therefore to fix the limits of the part in front. Unfortunately there seems to be no sort of unanimity with regard to the exact application of the names of the parts which follow, so that it is not easy to be sure to what parts the names should properly be applied, and it is extremely difficult to make comparisons between different forms without repeating a large part of the work already done by other observers. To take a concrete case: the part to which the name “œsophagus” has been applied has been limited, by different workers, by the extent of the stratified epithelium, by the extent of the striated muscle, by the absence of a serosa, by the absence of glands, and even by the relative diameter of the tube. Needless to say, these various boundaries do not coincide (in many cases some of them do not exist), and hence the difficulty of unravelling the descriptions in the literature of the subject is very great. So in what follows the names used will be defined, and I hope the nomenclature suggested will be such as to be acceptable and of scientific utility throughout the Chordata.