ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 305 



If an incision be now made along the line of the portal vein, and the gut pinned 

 out so as to expose the mucosa, it will be seen that the latter is only very slightly 

 attached to the submucous coat in preserved material (cp. Part II., fig. 1), with 

 the result that it very readily comes away. It is thrown into about ten prominent 

 longitudinal zigzag folds. Most of these folds are continued directly into those of 

 the cloaca, but dorsally, above the anus, the cloacal folds are independent structures, 

 and also ventrally there are smaller secondary folds developed between the larger 

 intestinal continuations. Very few of the folds of the mid-gut, in some specimens 

 none at all, pass straight from one extremity of the gut to the other. They may 

 bifurcate and join up again, so as to form an elongated loop, or they may bifurcate 

 without rejoining, which latter happens more commonly anteriorly than posteriorly. 

 Branches may be given off which themselves bifurcate, and this often occurs in 

 the region of the liver, so as to produce there a more complex pattern. 



Opposite the posterior extremity of the posterior lobe of the liver the gut begins 

 to change its calibre,* and the folds to flatten and die away, so that for a short 

 distance behind the opening of the bile duct the mucosa is thinner, and almost, but 

 never quite, smooth. However opposite, or even slightly behind, the biliary aperture 

 the folds rapidly increase in size and projection, and a short distance in front of the 

 opening they again project considerably into the lumen of the gut. The aperture 

 of the bile duct is situated directly in the course, and breaks the continuity, of one 

 of the ventral folds. 



The zigzagging of the folds of the intestine is often emphasised by very short 

 transverse folds, which project from the apex of one bend, and fit into, but never 

 join, the depression of a bend in the contiguous fold. The zigzags of neighbouring 

 folds therefore alternate. The pattern of the mucosa is, however, best seen by 

 removing it entire, which is only too easily done in the preserved gut, and examining 

 its submucous surface. We then notice that the same pattern is exhibited, only, 

 of course, in the form of a cast, in the submucosa itself, and here the short transverse 

 folds are indicated in a very striking manner. 



According to Maas (39), the section of the gut anterior to the abdominal intestine 

 may be separated into two regions — an anterior region, or true oesophagus, extending 

 from the ductus oesophago-cutaneus to the point of entrance of the gut into the body 

 cavity, and a posterior indiff"erent region, or "'stomach," the hinder boundary of which is 

 the opening of the bile duct. The former region has about six folds, and its lining 

 epithelium is stated by Maas to be of a character transitional between the many-layered 

 "ecto-" and the single-layered " endo-dermis," whilst the lining of the "stomach" 

 is truly endodermal, although its sub-mucosa is not that of typical mid-gut.t 



The gut, which has narrowed down considerably by the time the opening of the 

 bile duct is reached, is narrowest of all just in front of this opening, and it is here 

 closely invested by the cardiac portion of the M. constrictor branchiarum et cardise. 



* In most specimens it widens considerably here (cp. p. 311). t But cp. p. 312. 



