William A. HHton 463 



decide, but, to me, it seems doubtful that it indicates either a progress 

 towards or a remnant of, a perfect spiral valve. If it indicates a devel- 

 oping spiral valve, it would probably develop in a different manner, 

 as those of Elasmobranchs do. If a remnant of a once perfect spiral, 

 why do we not see more forms between man and fish having this pecu- 

 liar folding? 



Simple, primary ^ folds of the mucosa are easily distinguished from 

 secondary folds, such as the spiral valve and valvulge conniventes. The 

 simple folds are usually found when no villi are present, while the 

 spiral valve or valvule conniventes may have folds or villi upon them. 

 Then, too, the simple folds are more or less longitudinal in their dis- 

 tribution. They vary from thin or thick, slightly wavy to very wavy 

 or zigzag forms, and sometimes they may have a net-like arrangement 

 where some parts of the folds extend transversely. In no case do 

 transverse folds entirely replace the longitudinal ones. It might seem 

 from this that the original direction of the folds was longitudinal, but 

 in the course of development, parts of the folds in some cases had come 

 to take a transverse direction. 



A villus is a projection of the intestinal mucous membrane covered 

 with columnar epithelium and having its core made up of adenoid 

 connective tissue; it also has a network of capillaries in this core sup- 

 plied with one or more arterioles. In the central part of the villus 

 and surrounded by the blood vessels, are one or more lacteals. Usually 

 •a villus has its base diameter much less than its height, but there are 

 some exceptions to this. The distinction between a very broad villus 

 and a small fold cannot be a sharp one; usually, folds are compara- 

 tively few and continuous, while villi are distinct and very numerous. 



Villi are of many forms; they vary from thin plate-like kinds with 

 square or rounded edges much like parts of folds, to long or short, 

 slender or thick, cylindrical forms with rounded or sharp tips. 



For convenience, the occurrence of folds and villi in different animals 

 will be spoken of together. As other elevations of the mucosa have 

 been discussed as far as it is necessary for the purpose of this paper 

 they will simply be referred to occasionally. In many groups the con- 

 ditions existing in different species have been carefully described by 

 many investigators; besides giving a brief review of a few more im- 

 portant of the many forms studied by these, there will be added occa- 

 sional supplementary descriptions and descriptions of forms hitherto 

 undescribed. 



2 It seems best to consider as primary folds, those which are the simplest aiid the 

 first to develop. The secondary folds develop later and independently. 



